fix(list): keep sessions whose daemon outlives a socket-reachable failure

[schickling-assistant]

Closes #34.

listSessions() had two cleanup paths that fired even when the recorded
pid was still alive:

1. The .sock branch deleted the socket file whenever isSocketReachable
   returned false. The probe has a 500ms timeout and trips on a busy
   daemon, transient EAGAIN, or a race with a service restart. Once the
   .sock was gone, the still-alive daemon became invisible to every
   future scan and there was no way to recover it short of kill -9.
2. The .json branch unconditionally deleted metadata older than 24h.
   Long-lived daemons whose metadata wasn't refreshed (because nothing
   refreshes it) had their .json removed after a day even though they
   kept consuming memory.

This change gates both paths on isProcessAlive(pid):

• If the socket isn't reachable but the pid is alive, keep the .sock
  and report the session as running. Calling code that needs a
  reachable socket will fail in the usual way; the lister no longer
  destroys recovery state.
• If .json is older than 24h but the pid is alive, leave the metadata
  alone. The TTL is meant to age out known-dead sessions, not to garbage-
  collect unknown-state ones. The pid probe is cheap (a single
  kill(pid, 0) syscall) and runs at most once per dead-looking session
  per scan.

Tests in tests/list-filters.test.ts lock in both behaviors using the
test runner's own pid as a stand-in for an alive daemon, plus a
matched negative case proving the 24h TTL still fires when the pid is
dead.

Verification

• npx vitest run tests/list-filters.test.ts — all 18 tests pass
  (including the 2 new ones).
• npx vitest run tests/gc.test.ts tests/list-filters.test.ts — passes.
• npx tsc -p tsconfig.build.json — clean.

Posted on behalf of @schickling

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