fix: auto-close zombie sessions on queue owner shutdown (#47)

[manthan787]

Summary

• When the queue owner shuts down (TTL expired, no more tasks), auto-close the session if the agent process has already exited
• Prevents zombie session records from accumulating in ~/.acpx/sessions/ with closed=false despite the agent being dead
• Adds regression test that verifies sessions are properly closed after queue owner exit

Fixes #47

Details

The fix is in runSessionQueueOwner's finally block in src/session-runtime.ts. After releasing the queue owner lease, it reads the session record and marks it closed: true if lastAgentExitAt is set (indicating the agent has exited). This is best-effort — if the record is already cleaned up, the error is silently ignored.

The condition !record.closed && record.lastAgentExitAt ensures:

• Sessions already closed by sessions close are not double-written
• Sessions where the agent is still alive (edge case) are left open

Test plan

• New integration test: session auto-closes when queue owner exits and agent has exited (#47) — creates a session, sends a prompt with --ttl 1, waits for queue owner exit, verifies closed: true and closed_at are set on the session record
• All 150 existing tests continue to pass (including prompt reuses warm queue owner pid across turns which verifies the fix doesn't break warm queue owner behavior)

[vincentkoc]

Superseded by #70, which merged the queue-owner shutdown auto-close fix into main on 2026-03-09.