extensions/control: garbage-collect stale session-control sockets and aliases

[benvinegar]

Problem

pi/extensions/control.ts has an explicit TODO for stale socket/alias cleanup. Over long-lived runtimes, .pi/session-control can accumulate stale files from crashes or interrupted shutdowns.

Proposed solution

• Implement periodic GC in control extension startup (and/or heartbeat interval):
  • delete orphaned .alias symlinks
  • delete stale .sock entries not owned by a live process
  • optionally age-gate cleanup (e.g., older than 7 days)
• Add safety checks to avoid deleting active sockets.
• Add tests for stale/orphaned file handling.

Helpful context

• TODO marker in code: pi/extensions/control.ts around removeAliasesForSocket.
• Existing cleanup behavior is mainly in start.sh (best-effort on startup), but extension-level GC would keep state healthy during long uptimes.