Fix abandoned session cleanup

[sean174]

Summary

• Mark old unended sessions as abandoned_cleanup before pruning so abandoned rows do not bypass retention forever
• Wire the same finalize-then-prune behavior into hermes sessions prune and startup auto-maintenance
• Update session maintenance docs for abandoned-session cleanup behavior

Test Plan

• python -m pytest tests/test_hermes_state.py::TestAutoMaintenance tests/hermes_cli/test_sessions_delete.py -q
• python -m pytest tests/test_hermes_state.py tests/gateway/test_session_store_prune.py tests/gateway/test_session_state_cleanup.py tests/hermes_cli/test_sessions_delete.py -q
• python -m py_compile hermes_state.py hermes_cli/main.py hermes_cli/config.py
• git diff --check HEAD~1..HEAD

Notes

This is a defensive cleanup fix for long-lived/gateway installs where sessions may remain unended after crashes, restarts, or abandoned threads. Recently active sessions are protected by checking latest message timestamp, falling back to session start time for empty sessions.

[teknium1]

Thanks for addressing retention leaks in unended session rows. The underlying gap remains on current main: pruning selects only ended_at IS NOT NULL rows (hermes_state.py:5695, used by prune_sessions() at hermes_state.py:5867).

Problems

• The proposed last-message heuristic would change the current active-session contract. Current config says auto-prune only touches ended sessions (hermes_cli/config.py:3010-3014), and tests/test_hermes_state.py:2198-2209 requires a 200-day unended session to survive pruning. An idle but resumable gateway session is a concrete concern: recovery reopens durable rows in gateway/session.py:1391-1434.
• The CLI portion of c35eb54 is stale against the current candidate-preview flow. Current main builds filters, previews list_prune_candidates(), then confirms before prune_sessions() (hermes_cli/main.py:14367-14468). The new unended candidates need defined preview and confirmation semantics.

Suggested changes

• Use an authoritative lifecycle/liveness signal rather than message inactivity alone, and test that long-idle resumable sessions survive.
• Integrate the behavior into the current preview/confirmation flow and add gateway-recovery coverage.

Automated hermes-sweeper review.