Prune stale session records and tracking state

[ruban-24]

Context

Let It Brew stores session state in:

~/Library/Application Support/LetItBrew/sessions

Active sessions use one JSON file per session. Completed sessions are moved into timestamped tombstones under .locks so delayed lifecycle events cannot recreate an ended session. Per-session lock files also remain. Stop Tracking suppressions persist in UserDefaults until the same session produces a new working event.

Completed sessions are excluded from the once-per-second scan, and active records older than 12 hours cannot appear or keep the Mac awake. There is no confirmed memory leak.

The remaining problem is unbounded local storage:

• Tombstones accumulate as unique sessions end.
• A missed terminal event can leave an active record that is read and decoded every second, even after the 12-hour activity TTL excludes it.
• Stop Tracking suppressions can remain after their sessions disappear.
• Stale active records retain local project paths.

This is post-release hardening, not a release blocker.

Proposed behavior

Add conservative session maintenance that:

• Runs after launch and no more than once every 24 hours.
• Removes active records only when both the stored activity and unchanged file age exceed 24 hours.
• Retains terminal tombstones for at least 7 days before removing them.
• Removes Stop Tracking suppressions after 7 days when no matching session remains.
• Uses the existing per-session lock for every deletion.
• Never follows symlinks or removes files outside the exact sessions directory.
• Treats cleanup as best effort so a failure cannot interrupt polling or hook processing.
• Leaves the existing 12-hour activity TTL unchanged.

Do not delete per-session lock files in this change. Unlinking a lock file while another process holds it can split mutual exclusion across different file inodes. A fixed lock-shard design can be considered separately if lock-file growth becomes material.

Acceptance criteria

• Fresh active records are preserved.
• Active records older than the retention threshold are removed safely.
• Tombstones remain during the retention window and are removed afterwards.
• A concurrent newer lifecycle event wins over cleanup.
• Delayed events cannot recreate an eligible completed session.
• Symlinks, directories, malformed files, and unowned entries are never deleted.
• Expired Stop Tracking suppressions are removed only when their session is absent.
• Existing session-ordering and 100-session pressure tests continue to pass.
• Add focused cleanup and concurrency tests.
• Run the complete Swift test suite and a clean Xcode build.