fix(hindsight): implement on_session_end to flush buffered turns at session exit

[fayenix]

Summary

The hindsight memory provider buffers turns in-memory during a session but never flushes them when the session ends cleanly. This means buffered turns are silently lost if the gateway shuts down gracefully (no crash, no interrupt).

Changes

• Implement on_session_end() on the HindsightProvider to flush all buffered turns to the summary store when the gateway calls it during shutdown.
• Add test coverage for the new flush behavior (clean shutdown, empty buffer, error handling).

Testing

• Unit tests pass: pytest tests/plugins/memory/test_hindsight_provider.py
• Manual: confirmed buffered turns are persisted after graceful gateway stop.

Related

Resubmission of #16697 (closed without merge — commits preserved locally, re-cherry-picked onto current main).

[teknium1]

Thanks for chasing this; the premise is real on current main. Hindsight still buffers turns in sync_turn() until retain_every_n_turns and has no provider-specific on_session_end() on origin/main (plugins/memory/hindsight/__init__.py:1525; git grep only finds sync_turn() and on_session_switch()).

Problems

• The PR waits on self._sync_thread.join(timeout=10.0) before flushing (plugins/memory/hindsight/__init__.py:1512), but current Hindsight uses _sync_thread as a legacy alias for the long-lived writer thread (plugins/memory/hindsight/__init__.py:559-561, :957-978). That makes normal session-end flushing wait for the timeout instead of just draining queued work.
• The flush writes directly to self._document_id without _resolve_retain_target() or update_mode (plugins/memory/hindsight/__init__.py:1557). Current main’s retain path may use stable session_id plus update_mode='append' (plugins/memory/hindsight/__init__.py:1054-1073, :1530-1537), and legacy overwrite mode needs full-session content rather than only the modulo remainder.
• The added tests don’t cover that append/legacy split, document_id selection, or writer-queue ordering.

Suggested changes

• Mirror sync_turn()’s current retain-target rules in on_session_end() and use _last_retained_turn_count for append-mode deltas.
• Enqueue the final flush through the writer queue rather than joining the writer thread directly.

Automated hermes-sweeper review.