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-selection.ts

@@ -58,6 +58,7 @@ import {
   KeyringSecretStore,
   parseAccount,
   probeKeyringAvailable,
+  secretStoreGetMany,
   secretStoreGetStrict,
   secretStoreIsDurable,
   SessionSecretStore,
@@ -615,6 +616,21 @@ class DeferredSecretStore implements SecretStore {
   async isDurable(): Promise<boolean> {
     return secretStoreIsDurable(await this.target());
   }
+  /**
+   * Forwarded, and its absence was a real defect rather than an omission:
+   * this is the store production actually uses, so a `secretStoreGetStrict`
+   * call against it fell through to the tolerant `get` and the strictness
+   * existed only in tests that injected a concrete store.
+   */
+  async getStrict(serverId: string, field: string): Promise<string | null> {
+    return secretStoreGetStrict(await this.target(), serverId, field);
+  }
+  async getMany(
+    serverId: string,
+    fields: string[],
+  ): Promise<Record<string, string>> {
+    return secretStoreGetMany(await this.target(), serverId, fields);
+  }
   async get(serverId: string, field: string): Promise<string | null> {
     return (await this.target()).get(serverId, field);
   }