Add Consolidator.compact_idle_session(session_key, max_suffix=8) that
performs hard-truncation of idle sessions under the per-session
consolidation lock. This is the single lock-protected path for AutoCompact
to use instead of modifying session state directly, fixing the race
condition between AutoCompact and Consolidator.
Behavior:
- Acquires per-session consolidation lock
- Invalidates cache and reloads fresh from disk
- Splits unconsolidated tail into archive prefix and retained suffix
- Archives prefix via LLM (with raw_archive fallback on failure)
- Persists _last_summary in session metadata on success
- Returns summary text, None on LLM failure, or '' if nothing to archive
Tests: 6 new tests covering prefix archival, empty session timestamp
refresh, (nothing) summary exclusion, LLM failure fallback,
last_consolidated offset, and lock acquisition verification.
48d35bd2d9