feat(coding-agent): reap orphaned empty session files at daemon start

[y0usaf]

When the daemon (or a worker) gets killed mid-flight, it can leave a sessions/*.jsonl behind that never captured a single message — just the header and a few model/state bookkeeping lines. They pile up and clutter the session list.

This cleans those up: on daemon start it scans the session dir and deletes any file whose entries are purely bookkeeping. Things that actually contain a message, a custom entry, or a label are left alone, and any session a live process still holds is skipped (via a new hasActiveSessionLease() helper). Deletion goes through the existing trash-first path, so it's recoverable.

Small, self-contained, no test balloon — code speaks for itself.

Note

Reap orphaned empty session files on daemon startup

• On startup, AgentDaemon now calls a new reapOrphanedEmptySessions method that scans .jsonl session files and deletes those containing only bookkeeping entries (e.g. session, model_change, agent_status) with no active lease.
• A new hasActiveSessionLease function in session-lease.ts checks whether a session file's lease owner process is still alive, protecting files from active sessions.
• Files that are unreadable or corrupt are silently skipped to avoid blocking startup.

^{Macroscope summarized f50fa1d.}