Add src/poller.rs with FIFO O_NONBLOCK open (read-end + keeper write-end),
Stop hook JSON payload parsing, transcript path derivation via cwd slug,
and StopInfo resolution. Wire poller into EventLoop via add_fifo_fd() which
was already present in event_loop.rs from Phase 3.
Update mock-claude to emit proper JSON Stop payloads (with and without
transcript_path via MOCK_OMIT_TRANSCRIPT_PATH=1) and update the pty_integration
assertion to match.
Tests test_stop_hook_fires and test_missing_transcript_path_derived both pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
