fix(auth): address Copilot review round 1 on the secrets-file lock (#2082)

- Create the storage directory before locking. `writeStoreFile` creates it,
  but from inside the locked section — so on a fresh install, where
  `~/.mcp-inspector` does not exist, proper-lockfile failed ENOENT and every
  *first* save degraded to an unlocked write with a warning. That is the save
  most likely to be racing another (two Inspectors started together both
  reach it), so the lock was absent from precisely the interleaving it exists
  to close.
- `anythingToMigrate` no longer reads every `readdir` failure as "nothing to
  migrate". Only ENOENT/ENOTDIR proves the fresh-install case; a directory
  that denies listing can still permit access to the known `secrets.json`,
  and returning false there selected the keychain and left those secrets
  invisible with nothing said. Other errors fall through to the under-lock
  checks, which is what happened before the fast path existed.
- Pin `proper-lockfile` to the repo-root install in `vitest.shared.mts`,
  beside express and yaml: it is reached only through root-owned `core/`.
  Resolution already finds the root copy since no client declares it; the pin
  is what stops that depending on it never arriving as some client's
  transitive dependency, which would give a test two copies of a module whose
  whole job is one registry of held locks.
- Make the cross-process test prove `mutate` takes the lock. The previous one
  asserted only that both secrets survived, which the optimistic verify
  delivers with the lock removed entirely. It now asserts the parent's `set`
  has *not* settled while a child holds the lock — an observation only a real
  lock produces. Verified by removing `withSecretFileLock` from `mutate`:
  three tests fail, where none did before.

The degrade test's "lock cannot be created" case is now a path whose parent
is a *file* rather than a missing directory, since a missing directory is no
longer that case. It also fails identically for root, so it cannot pass
locally and flake in a container.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JTHVxSu8AUgHRLvo1ntZ8H
Signed-off-by: cliffhall <cliff@futurescale.com>

vitest.shared.mts

@@ -101,6 +101,17 @@ export function vitestSharedPaths(clientDir: string) {
       find: /^yaml$/,
       replacement: path.resolve(repoRoot, "node_modules/yaml"),
     },
+    // Same reasoning, one layer in: `proper-lockfile` is reached only through
+    // `core/` (the secrets file's cross-process lock, #2082), which is the
+    // other root-owned tree with no manifest of its own. Resolution finds the
+    // root copy on its own today — nothing declares it in a client — and this
+    // pin is what keeps that from depending on nothing ever arriving as some
+    // client's transitive dependency, which would otherwise give a test two
+    // copies of a module whose whole job is a single registry of held locks.
+    {
+      find: /^proper-lockfile$/,
+      replacement: path.resolve(repoRoot, "node_modules/proper-lockfile"),
+    },
   ];
 
   const projectResolve = {