feat: per-agent status tracking with Python watcher daemon

[jezekra1]

Summary

• Per-agent fact files: Each agent (main + subagents) writes its own {session_id}.agent.{key}.json file. File locking (fcntl.flock) prevents concurrent hook races. Session-level status is derived from agent priorities: needs_approval > working > waiting.
• Python watcher daemon (--watch mode): Single background process polls every 1.5s, resolving hook blind spots:
  • Transcript resolution: detects tool completion/rejection when no hook fires
  • Process inspection: flips needs_approval → working when a command is already running as a child process
  • Interruption detection: sets all agents to waiting on user interrupt
  • PID liveness cleanup: removes dead session files
  • Self-terminates after 60s with no active sessions. ensure_watcher() on every hook invocation restarts it if needed.
• Swift simplified to pure file reader: SessionProcessor stripped from 273 → 65 lines (only deriveStatus, isGhost, abandoned detection). SessionStateWatcher uses DispatchSource only — no timer, no PID checks, no transcript parsing. All resolution state is persisted to disk.
• Lock file cleanup: fact file deletion now also removes .lock files.
• Bug fix: derive_agent_status returned stale status when tools dict was empty instead of "waiting".
• Sessions start in "waiting" state: Fixes resumed sessions falsely showing as "working".
• StopFailure hook added, unreliable Notification handlers removed.

Test plan

• swift build passes
• uv run pytest Tests/python/ -v — 61/61 tests pass (28 new tests for agent facts, watcher resolution, lifecycle simulation)
• Start a new Claude session — should show "your turn" until user sends a message
• Reject a tool — should update to "your turn" within ~2s (watcher resolves via transcript)
• Approve a slow command (e.g. sleep 20) — should transition to "working" immediately (watcher process inspection)
• Interrupt a session (Ctrl+C) — all agents should show "your turn"
• Run a session with subagents — statuses should be independent
• Kill a Claude process — watcher should clean up the session within 1.5s