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>

core/auth/node/secret-store.ts

@@ -337,6 +337,20 @@ export interface SecretStore {
    * absent, which is correct for stores whose reads cannot fail.
    */
   getStrict?(serverId: string, field: string): Promise<string | null>;
+  /**
+   * Read several of one server's fields in a single pass.
+   *
+   * Optional, and purely a performance seam: {@link secretStoreGetMany}
+   * falls back to a parallel `get` per field, which is what the keychain
+   * wants anyway (independent native round-trips).
+   *
+   * It exists for `FileSecretStore`, where the shape is inverted — every
+   * `get` reads and decrypts the *whole* file, so N fields cost N scrypt
+   * derivations, serialized behind the store's own queue. The rehydration
+   * callers ask for one server's fields at a time, so a per-server bulk read
+   * turns that back into one.
+   */
+  getMany?(serverId: string, fields: string[]): Promise<Record<string, string>>;
   set(serverId: string, field: string, value: string): Promise<void>;
   /** No-op if no entry exists. */
   delete(serverId: string, field: string): Promise<void>;
@@ -514,6 +528,29 @@ export async function secretStoreGetStrict(
     : store.get(serverId, field);
 }
 
+/**
+ * Read several fields of one server, using the store's bulk path when it has
+ * one. Returns only the fields that are present, matching the callers that
+ * merge the result into a stored config.
+ */
+export async function secretStoreGetMany(
+  store: SecretStore,
+  serverId: string,
+  fields: string[],
+): Promise<Record<string, string>> {
+  if (store.getMany) return store.getMany(serverId, fields);
+  const entries = await Promise.all(
+    fields.map(
+      async (field) => [field, await store.get(serverId, field)] as const,
+    ),
+  );
+  const out: Record<string, string> = {};
+  for (const [field, value] of entries) {
+    if (value !== null) out[field] = value;
+  }
+  return out;
+}
+
 export async function secretStoreIsDurable(
   store: SecretStore,
 ): Promise<boolean> {