fix(auth): address Copilot review round 8 (four inline findings)

The lock finding from this round is deliberately not addressed here — see
below.

- **My round-6 strict-read fix was inert in production.** `DeferredSecretStore`
  is what `defaultSecretStore()` returns, and it did not forward `getStrict`
  — so `secretStoreGetStrict` fell through to the tolerant `get` for every
  real caller, and the strictness existed only in tests that injected a
  concrete store. Forwarded now, along with `getMany`.

- **The same tolerant-read bug remained in both pre-existing migrations.**
  I added `getStrict` for the new hand-off in round 6 and never applied it to
  the mcp.json and client.json migrations, which have the identical
  shape: decide from `get`, then write on `null`. A transient keychain read
  failure therefore let an older plaintext value overwrite a newer keychain
  one — and the disk copy was then stripped, so the newer value was gone. Both
  use the strict read now; the throw is a `SecretStoreUnavailableError`, which
  both call sites already handle as "abandon the migration, keep the file".

- **The permission check made a false statement on Windows.** `stat` returns
  synthetic bits there, and interpreting them yields a confident falsehood
  either way — "0666 — not owner-only" about a file the ACL may restrict, or
  "owner-only" about one it does not. The ACL is the real answer and this does
  not inspect it, so Windows now reports `unknown`.

- **Rehydration cost one full decryption per field.** `get` reads and decrypts
  the entire file, and both rehydration paths ask field by field, serialized
  behind the store's own queue. Measured on this machine, scrypt at N=16384 is
  ~23ms per derivation, so a 20-secret encrypted catalog spent **~450ms of
  pure key derivation on every `GET /api/servers`** — a visible stall, not a
  micro-optimization. Added an optional `getMany` seam: `FileSecretStore`
  decrypts once per server, everything else falls back to the parallel
  per-field `get` that already suited it. A test counts derivations directly:
  1 for a three-field `getMany`, 3 for the three `get`s it replaced.

Also broadens a log message that still said "Keychain unavailable" for a
store that may not be a keychain.

Full `npm run ci` green — 5,762 tests under the coverage gate.

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

clients/web/src/test/core/client/node-persistence.test.ts

@@ -63,6 +63,35 @@ describe("client node-persistence", () => {
     ).toBe("plain");
   });
 
+  it("aborts rather than overwriting when the keychain read fails", async () => {
+    // The keychain-wins lookup used the tolerant `get`, which maps an
+    // unreadable store to `null` — and the branch below writes on `null`. So
+    // a transient read failure let the older client.json copy overwrite a
+    // newer stored secret, and the disk copy was then stripped.
+    const filePath = await makeTmpFile(
+      JSON.stringify(configWithPlaintextSecret),
+    );
+    let wrote = false;
+    const flaky: SecretStore = {
+      get: async () => null,
+      getStrict: async () => {
+        throw new KeychainUnavailableError(new Error("temporarily down"));
+      },
+      set: async () => {
+        wrote = true;
+      },
+      delete: async () => {},
+      deleteAllForServer: async () => {},
+    };
+
+    const loaded = await readClientConfigStore(filePath, flaky);
+
+    expect(wrote).toBe(false);
+    // The plaintext survives for the next attempt.
+    expect(readFileSync(filePath, "utf-8")).toContain("plain");
+    expect(loaded.enterpriseManagedAuth?.idp.clientSecret).toBe("plain");
+  });
+
   it("keeps the plaintext on disk when the store is session-scoped", async () => {
     // The container fallback. Migrating here would delete a secret that
     // survives restarts and keep only a copy that dies with the process —