Recording Phase 2: Markerized Timeline and markers API

[mark-e-deyoung]

Context: Phase 1 added recording timeline IDs and artifact manifests.\n\nScope:\n- Emit explicit markers for start, pause, resume, stop.\n- Emit markers for control-mode transitions and human/agent handoff events.\n- Persist markers to recording_markers.jsonl in the active session.\n- Add GET /recording/markers with filters for session_id, session_dir, timeline_id, and limit.\n- Include recording_timeline_id and both wall-clock plus monotonic timestamps per marker.\n\nAcceptance criteria:\n- Unit and API contract tests cover marker emission and retrieval.\n- E2E verifies marker order and correlation with UI and API transitions.\n- Docs updated in docs/api.md and docs/recording.md.