In `@src/session/registry.ts`:
- Around line 80-116: The registry directory and file are created with default
permissive Unix modes; update ensureRegistryDir to call mkdir with mode: 0o700
(and keep recursive: true) and update saveRegistry to write the file with a
restrictive mode: 0o600 when calling writeFile; additionally, after creation (in
ensureRegistryDir and saveRegistry) ensure existing items are corrected by
calling fs.chmod on REGISTRY_DIR and the path returned by getRegistryPath() to
enforce 0o700 and 0o600 respectively so permissions are correct even if the
directory/file already existed.
- Around line 113-152: The registry mutations (registerSession,
updateRegistryStatus, unregisterSession) must acquire a file-level lock around
the read-modify-write sequence to avoid concurrent clobbering; implement a lock
(advisory flock or lockfile) around loadRegistry -> modify registry.sessions ->
saveRegistry and ensure the lock is always released even on errors. Replace the
current writeFile usage in saveRegistry with an atomic-write pattern: write JSON
to a temp file in the same directory as getRegistryPath(), fsync the temp file
(and its parent dir if possible), then rename the temp file over the target path
to ensure atomic replace and durability; ensure errors during write/rename clean
up the temp file. Keep ensureRegistryDir and getRegistryPath for locating files,
and integrate locking/unlocking with these functions so all registry mutations
are serialized across processes.

In `@src/session/registry.ts`:
- Around line 175-192: The validation in loadRegistryInternal accepts null or
arrays because it only checks typeof parsed.sessions === 'object'; update the
condition to ensure parsed.sessions is a plain, non-null object (e.g.,
parsed.version === 1 && parsed.sessions && !Array.isArray(parsed.sessions)
and/or check Object.prototype.toString.call(parsed.sessions) === '[object
Object]') so that loadRegistryInternal returns a clean { version: 1, sessions:
{} } when sessions is null/array/other non-plain types; adjust the parsed
validation and keep the same fallback behavior.
- Around line 122-154: The acquireLock loop currently swallows all errors from
unlink(lockPath) when attempting to remove a stale lock, which allows an
infinite hang if unlink fails (e.g., read‑only FS); modify the stale‑lock branch
inside acquireLock to capture the unlink error and if unlink fails with anything
other than ENOENT (or if unlink fails after LOCK_TIMEOUT_MS) rethrow or return a
clear error so the caller won't loop forever; reference acquireLock, lockPath,
unlink, LOCK_TIMEOUT_MS and LOCK_RETRY_DELAY_MS when making this change.
- Around line 365-381: The current cleanupStaleRegistryEntries holds
withRegistryLock while awaiting checkSessionExists for each registry.sessions
entry; instead, snapshot registry.sessions (map sessionId -> cwd), release the
lock, run checkSessionExists for each cwd and collect stale sessionIds, then
reacquire withRegistryLock and in a short critical section delete only those
sessionIds whose registry.sessions[sessionId] still exists and still has the
same cwd (to avoid race) and increment cleaned; reference functions/values:
cleanupStaleRegistryEntries, withRegistryLock, registry.sessions,
checkSessionExists, and ensure LOCK_TIMEOUT_MS is not hit by long checks being
done outside the lock.

In `@src/commands/resume.tsx`:
- Around line 193-227: When resolving a session by registry ID in
resolveSession, validate that the session's backing file actually exists before
returning the registry entry: after you obtain the SessionRegistryEntry (from
getSessionById or findSessionsByPrefix), check the session file path referenced
by entry (e.g. entry.sessionFile or entry.cwd-derived path) and if the file is
missing log a clear, specific message (same cleanup guidance used in the generic
load failure path) prompting the user to remove the stale registry entry or
recreate the session, then return null instead of returning the stale
registryEntry; update logging to include args.sessionId and reuse the existing
user-facing cleanup guidance text.