Add a turn-liveness watchdog keyed to the agent activity clock: a turn
that stalls mid-flight while the durable lease keeps renewing is logged
loudly, surfaced to the UI, force-interrupted, and — when the hard
interrupt cannot unwind the wedge — lease renewal is stopped so
stale-turn cleanup can reclaim the session.
Race safety (rounds 3/4/6 of the #95663 review, all folded into this
squashed commit):
- AIAgent.interrupt(require_generation=G) re-validates the generation
claim at the last instant before the hammer; a stale claim abandons
the abort and the turn continues.
- The claim is reserved under the activity lock, invalidated by any real
progress in _touch_activity(), and consumed immediately before the
first observable interrupt publication; exceptional paths fail closed.
- Claim consumption and the first interrupt publication are atomic
inside one _liveness_activity_lock() critical section; unclaimed
interrupts publish lock-free so AIAgent stand-ins without the liveness
seam keep working (CI 33096454629 regression, fixed here).
Deterministic race regressions (written red-first) in
tests/run_agent/test_turn_liveness_watchdog.py cover the
post-revalidation window, the consume-to-publication window, the
exceptional path, and atomic claim consumption.
Round-7 rebuild: single squashed commit on current origin/main; the
former four-commit lineage (241f8e484..299122558 on merge base
6defe7eb6c) no longer exists, so no surviving commit carries a red
exact-object CI record, and no empty CI-trigger commit was added.
Finn763