chore(agent): tripwire — warn when a turn starts before the previous turn's persist

[Rival]

#64934 documents two turns running concurrently on one gateway session
(busy-guard miss, route unidentified): interleaved flushes persisted user
rows out of arrival order and swallowed an assistant row, leaving a
permanent alternation wedge.

This adds a log-only tripwire: note_turn_start in build_turn_context
warns — naming both turn_ids, the session, and the previous turn's age —
when a turn starts before the previous turn's _persist_session completed;
note_turn_persisted clears the slot in the persist funnel (both lock
branches). The WARNING's two turn_ids correlate with dispatch logs, so the
next occurrence in any deployment identifies the route that bypassed the
busy guard.

Semantics tuned against noise: ownership transfer keeps a turn that crashed
before its persist from warning more than once; the unconditional clear
makes the tripwire under-report rather than double-report under a real
overlap; same-turn_id re-entry (retry paths) is silent. No behavior change.

4 unit tests (tests/agent/test_turn_overlap_tripwire.py); full
tests/agent/ suite green (5723 tests).

🤖 Generated with Claude Code

[teknium1]

Thanks for adding a focused diagnostic for the concurrency report. The current implementation needs a lifecycle correction before it can reliably identify the bypass route.

Problems

• note_turn_persisted() is wired into every AIAgent._persist_session() call, but build_turn_context() already calls _persist_session() for crash-resilient turn-start persistence at agent/turn_context.py:379-389. That clears the marker before the pre-LLM work and conversation loop complete, so an overlapping turn starting after that initial flush is silent. agent/conversation_loop.py also has multiple mid-turn persistence calls.
• tests/agent/test_turn_overlap_tripwire.py tests helpers in isolation, so it does not cover the turn-start-flush → overlap sequence above.

Suggested changes

• Clear the diagnostic only at a genuine terminal turn boundary (the normal finalizer persists at agent/turn_finalizer.py:240) and cover required terminal error paths.
• Add an integration test that blocks turn A after its initial persistence, starts turn B, and asserts that the warning is emitted.

Automated hermes-sweeper review.

[teknium1]

Merged via PR #65499 — your commit was cherry-picked onto current main with authorship preserved. The noise-tuning semantics (ownership transfer on crashed turns, unconditional clear so overlaps under-report, silent same-turn re-entry) survived review intact; we E2E-verified the persist-funnel clear in both lock branches.

Next occurrence of the #64934 overlap in any deployment should now name both turn_ids in the WARNING — please share the correlated dispatch log lines on the issue when it fires.