* feat(W-024): reboot_event audit trail entry (L10b.3, ADR-0158)
L10 scope §Sub-question 3: a reboot_event analog of TurnEvent, written
to the telemetry JSONL, lets future audit reconstruct when this engine
instance lost and regained its lifetime.
- serialize_reboot_event / format_reboot_event_jsonl in chat/telemetry.py
emit type="reboot" with restored_turn_count, stored/current revisions,
revision_matched, recognizers_count, candidates_count
- ChatRuntime._load_engine_state() buffers the JSONL line in
_pending_reboot_payload (str|None); ChatRuntime.attach_telemetry_sink()
flushes it exactly once when a sink is first attached
- Reboot event precedes all turn events in the session audit stream
- Pinned by 11 tests: serializer structure, determinism, revision_matched
logic, runtime integration (emit-once, no-checkpoint, no-load-state,
revision match, ordering)
Closes L10b: W-022 (atomic writes) + W-023 (revision warning) + W-024
together satisfy ADR-0146's atomic/observable/auditable checkpoint triad.
* fix(W-024): expose cached public git revision helper
