feat(daemon): log checkpoint admission delayed behind trace ingestion

[svarlamov-git-ai]

Summary

Observability follow-up for #2252 (stacked on the fix PR), implementing the issue's third suggested direction:

Emit an explicit log line when ingress admission is delayed beyond a threshold (seq 77-84 above had to be inferred from the absence of admission lines).

Checkpoint admission waits on the trace-ingest watermark. In the incident, checkpoints sat "received into bounded ingress" with no admission line, and the stall had to be inferred from absent logs. Admission now logs a warning once the wait exceeds 5 seconds (CHECKPOINT_ADMISSION_DELAY_LOG_INTERVAL), repeating at that cadence while it persists:

WARN checkpoint admission delayed waiting for trace ingestion component="daemon" phase="checkpoint_admission" reason="admission_delayed_by_trace_ingest" receipt_seq=77 trace_ingest_target=1042 processed_trace_ingest_seq=1017 waited_ms=5000

The healthy path is unaffected: wait_for_trace_ingest_seq checks the watermark atomically before enrolling in the notify, so when admission completes in milliseconds the 5s timer is never observably involved, and the wrapped wait is safe to cancel/retry (the inner loop re-enrolls before every re-check, so no wakeups are lost; shutdown still exits the loop).

The interval is overridable in test builds via GIT_AI_TEST_CHECKPOINT_ADMISSION_DELAY_LOG_INTERVAL_MS so the test below keeps a wide margin on slow CI runners instead of racing daemon startup against the 5s threshold.

Tests (TDD)

• delayed_checkpoint_admission_is_logged — stalls the ingest worker with the existing GIT_AI_TEST_TRACE_INGEST_WORKER_START_DELAY_MS hook, enqueues synthetic mutating trace frames, submits a checkpoint, and asserts the warning appears. Fails without this change.