fix(auth): address Copilot review on the secret-store PR

Six findings, all taken.

- The descriptor reported the *write policy*, not the file. Adding
  `MCP_INSPECTOR_SECRET_KEY` to an install that already has a plaintext
  file flipped `store.encrypted` immediately, so the banner and footer
  claimed "File (encrypted)" while the existing secrets sat readable
  until the next `set`. `FileSecretStore.readOnDiskEncryption()` now
  reads the envelope (never the payload, so it needs no passphrase) and
  the descriptor is built from that. The transitional state gets its own
  `pendingEncryption` flag, which changes the advice without softening
  the verdict — "set a passphrase" is the wrong thing to tell someone who
  already has, so the caveat names the pending write instead.

- The secrets file ignored the configured storage directory. A container
  mounting only that directory was judged unmounted, fell back to
  `memory`, and lost secrets the user had arranged to keep.
  `defaultSecretFilePath()` now follows `MCP_STORAGE_DIR` — the variable
  that already relocates OAuth tokens and `client.json` — between the
  explicit `MCP_INSPECTOR_SECRET_FILE` and the `~/.mcp-inspector`
  default. (The issue comment called it `MCP_INSPECTOR_STORAGE_DIR`; no
  such variable exists.)

- `version` was never checked before the plaintext branch, so a
  `{ version: 2, encryption: "none" }` file was read and silently
  rewritten as version 1, discarding whatever the newer format added —
  the same destroy-to-satisfy-an-additive-request case the encrypted
  branch already refuses. Checked first now, so the answer no longer
  depends on which encryption mode the newer writer chose.

- The doc comment claimed a per-file salt made "a short, memorable value
  safe to use". It does not: the salt defeats precomputation, not
  guessing, and the scrypt cost is deliberately a few milliseconds
  because it runs on every read and write. Both the comment and the
  README now say to use a high-entropy value, generated rather than
  chosen.

- `usableSecretStorage` validated only `kind`, so a partial
  `{ kind: "file" }` was cast through and — because a missing `plaintext`
  is falsy — rendered as the quiet, neutral *encrypted* file. That is the
  confident half-answer the guard exists to reject, produced by the
  absence of information rather than by any claim the backend made.
  `reason`, `durable`, and each kind's own required fields are checked.

- The footer's inline `Tooltip` carried four static props; extracted to a
  `CaveatTooltip` constant per the `.withProps()` rule.

Tests cover each: the policy/file disagreement in both directions and the
no-file-yet fallback, `MCP_STORAGE_DIR` and its precedence against
`MCP_INSPECTOR_SECRET_FILE`, the version refusal (asserting the bytes
survive it) split from the unrecognized-cipher refusal it used to be
conflated with, `readOnDiskEncryption` including the keyless and
unparseable cases, five new rejected descriptor shapes, and the changed
caveat. Full `npm run ci` green.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/integration/auth/node/file-secret-store.test.ts

@@ -225,6 +225,31 @@ describe("FileSecretStore failure handling", () => {
     );
   });
 
+  it("refuses a file written by a newer format version", async () => {
+    // Readable as far as the plaintext branch is concerned, which is the trap:
+    // accepting it means the next write rewrites it as version 1 and silently
+    // drops whatever the newer format added. Same refusal the encrypted branch
+    // already gives, so the answer doesn't depend on which mode the newer
+    // writer happened to choose.
+    await fs.writeFile(
+      filePath(),
+      JSON.stringify({
+        version: 2,
+        encryption: "none",
+        secrets: { "a:b": "1" },
+      }),
+      "utf-8",
+    );
+    const store = new FileSecretStore({ filePath: filePath() });
+    expect(await store.get("a", "b")).toBe(null);
+    await expect(store.set("a", "b", "2")).rejects.toThrow(/version 2/);
+    // The bytes are still there — nothing was destroyed to answer the write.
+    const raw = JSON.parse(await fs.readFile(filePath(), "utf-8")) as {
+      version: number;
+    };
+    expect(raw.version).toBe(2);
+  });
+
   it("delete stays silent on a file it cannot decrypt", async () => {
     await writeEncryptedFixture();
     const store = new FileSecretStore({
@@ -246,15 +271,34 @@ describe("FileSecretStore failure handling", () => {
     expect(await store.get("alpha", "env:A")).toBe(null);
   });
 
-  it("set reports a file written in a format this build doesn't know", async () => {
+  it("set reports a file written by a newer format version", async () => {
     // The forward-compat case: a newer Inspector's file, opened by an older
     // one. Refusing beats truncating it to the entries we happen to parse.
+    // The version is checked before the encryption mode, so this is the
+    // message even though the cipher is also unrecognized — the version is
+    // the more actionable of the two ("upgrade"), and it is the reason the
+    // file is unreadable regardless of what the cipher turned out to be.
     await fs.writeFile(
       filePath(),
       JSON.stringify({ version: 99, encryption: "chacha20-poly1305" }),
       "utf-8",
     );
     const store = new FileSecretStore({ filePath: filePath() });
+    await expect(store.set("alpha", "env:A", "1")).rejects.toThrow(
+      /format version 99/,
+    );
+  });
+
+  it("set reports an unrecognized cipher at a version it does understand", async () => {
+    // The other half: a file this build's version check accepts, whose
+    // encryption mode it has no code for. A different failure with different
+    // advice, so it must not collapse into the version message.
+    await fs.writeFile(
+      filePath(),
+      JSON.stringify({ version: 1, encryption: "chacha20-poly1305" }),
+      "utf-8",
+    );
+    const store = new FileSecretStore({ filePath: filePath() });
     await expect(store.set("alpha", "env:A", "1")).rejects.toThrow(
       /unsupported format/,
     );
@@ -409,3 +453,48 @@ describe("tightenSecretFilePermissions", () => {
     ).resolves.toBeUndefined();
   });
 });
+
+describe("readOnDiskEncryption", () => {
+  it("reports what the file is, not what the store would write", async () => {
+    // The whole reason it exists: these two disagree for a full session after
+    // a passphrase is added to an install that already has a plaintext file.
+    const plain = new FileSecretStore({ filePath: filePath() });
+    await plain.set("alpha", SECRET_FIELD_OAUTH_CLIENT_SECRET, "shh");
+
+    const withKey = new FileSecretStore({
+      filePath: filePath(),
+      passphrase: "hunter2",
+    });
+    expect(withKey.encrypted).toBe(true);
+    expect(await withKey.readOnDiskEncryption()).toBe("none");
+
+    await withKey.set("alpha", "env:A", "1");
+    expect(await withKey.readOnDiskEncryption()).toBe("aes-256-gcm");
+  });
+
+  it("answers null for an absent, unparseable, or unrecognized file", async () => {
+    const store = new FileSecretStore({ filePath: filePath() });
+    expect(await store.readOnDiskEncryption()).toBe(null);
+
+    await fs.writeFile(filePath(), "{not json", "utf-8");
+    expect(await store.readOnDiskEncryption()).toBe(null);
+
+    await fs.writeFile(
+      filePath(),
+      JSON.stringify({ version: 1, encryption: "rot13" }),
+      "utf-8",
+    );
+    expect(await store.readOnDiskEncryption()).toBe(null);
+  });
+
+  it("needs no passphrase — it reads the envelope, never the payload", async () => {
+    // A store whose key is wrong still has to be describable in the UI.
+    const writer = new FileSecretStore({
+      filePath: filePath(),
+      passphrase: "right-key",
+    });
+    await writer.set("alpha", SECRET_FIELD_OAUTH_CLIENT_SECRET, "shh");
+    const keyless = new FileSecretStore({ filePath: filePath() });
+    expect(await keyless.readOnDiskEncryption()).toBe("aes-256-gcm");
+  });
+});

clients/web/src/test/integration/auth/node/secret-store-selection.test.ts

@@ -33,6 +33,7 @@ const ENV_KEYS = [
   "MCP_INSPECTOR_SECRET_STORE",
   "MCP_INSPECTOR_SECRET_FILE",
   "MCP_INSPECTOR_SECRET_KEY",
+  "MCP_STORAGE_DIR",
   "KUBERNETES_SERVICE_HOST",
 ];
 
@@ -381,4 +382,86 @@ describe("defaultSecretFilePath", () => {
       path.join(".mcp-inspector", "secrets.json"),
     );
   });
+
+  it("follows MCP_STORAGE_DIR when no explicit file is named", async () => {
+    // The variable that already relocates OAuth tokens and client.json. A
+    // container mounting only that directory must not be judged unmounted and
+    // demoted to `memory` — the user arranged for exactly this directory to
+    // survive.
+    process.env.MCP_STORAGE_DIR = tmpDir;
+    const mod = await loadWithProbe(true);
+    expect(mod.defaultSecretFilePath()).toBe(path.join(tmpDir, "secrets.json"));
+  });
+
+  it("lets an explicit MCP_INSPECTOR_SECRET_FILE outrank MCP_STORAGE_DIR", async () => {
+    // Naming the file outright is the more specific statement of the two.
+    process.env.MCP_STORAGE_DIR = tmpDir;
+    process.env.MCP_INSPECTOR_SECRET_FILE = path.join(tmpDir, "elsewhere.json");
+    const mod = await loadWithProbe(true);
+    expect(mod.defaultSecretFilePath()).toBe(
+      path.join(tmpDir, "elsewhere.json"),
+    );
+  });
+});
+
+describe("the descriptor reports the file, not the write policy", () => {
+  it("says plaintext while a pre-existing plaintext file is only *going* to be encrypted", async () => {
+    // The transitional state: the passphrase is set, so the next write will
+    // encrypt, but the bytes on disk are still readable. Reporting "File
+    // (encrypted)" here is the one false reassurance this subsystem exists to
+    // avoid.
+    const filePath = path.join(tmpDir, "secrets.json");
+    process.env.MCP_INSPECTOR_SECRET_FILE = filePath;
+    process.env.MCP_INSPECTOR_SECRET_STORE = "file";
+    vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    const plain = await loadWithProbe(false);
+    await plain.defaultSecretStore().set("alpha", "env:A", "1");
+
+    // Same file, now with a passphrase configured.
+    process.env.MCP_INSPECTOR_SECRET_KEY = "hunter2";
+    const withKey = await loadWithProbe(false);
+    const info = await withKey.getSecretStorageInfo();
+    expect(info.plaintext).toBe(true);
+    expect(info.pendingEncryption).toBe(true);
+
+    // And the advice changes with it: telling someone to set a passphrase
+    // they have already set is worse than saying nothing.
+    const { secretStorageCaveat, secretStorageLabel } =
+      await import("@inspector/core/auth/secret-storage-info.js");
+    expect(secretStorageLabel(info)).toBe("File (unencrypted)");
+    expect(secretStorageCaveat(info)).toMatch(/next time a secret is saved/);
+  });
+
+  it("says encrypted once the upgrading write has actually landed", async () => {
+    const filePath = path.join(tmpDir, "secrets.json");
+    process.env.MCP_INSPECTOR_SECRET_FILE = filePath;
+    process.env.MCP_INSPECTOR_SECRET_STORE = "file";
+    vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    const plain = await loadWithProbe(false);
+    await plain.defaultSecretStore().set("alpha", "env:A", "1");
+
+    process.env.MCP_INSPECTOR_SECRET_KEY = "hunter2";
+    const withKey = await loadWithProbe(false);
+    await withKey.defaultSecretStore().set("alpha", "env:B", "2");
+
+    const after = await loadWithProbe(false);
+    const info = await after.getSecretStorageInfo();
+    expect(info.plaintext).toBe(false);
+    expect(info.pendingEncryption).toBeUndefined();
+  });
+
+  it("falls back to the write policy when there is no file yet", async () => {
+    // Nothing on disk to describe, and the policy is then accurate: the first
+    // write creates the file in exactly that mode.
+    process.env.MCP_INSPECTOR_SECRET_FILE = path.join(tmpDir, "absent.json");
+    process.env.MCP_INSPECTOR_SECRET_STORE = "file";
+    process.env.MCP_INSPECTOR_SECRET_KEY = "hunter2";
+    vi.spyOn(console, "warn").mockImplementation(() => {});
+    const mod = await loadWithProbe(false);
+    const info = await mod.getSecretStorageInfo();
+    expect(info.plaintext).toBe(false);
+    expect(info.pendingEncryption).toBeUndefined();
+  });
 });