feat(memory): add turn-based distillation trigger alongside idle timeout

[Aaronontheweb]

Summary

Closes #442

• Adds MemoryDistillationTurnInterval tuning property (default 5) to trigger memory distillation every N completed turns, regardless of idle state
• Busy sessions with long tool loops (e.g., 25-turn scanner iterations) now form memories during active use instead of waiting for 90s of silence
• The idle timeout (MemoryObserverIdleSeconds) remains as a fallback for partial-turn content
• Set to 0 to disable turn-based triggering

Test plan

• All existing observer, distillation, watchdog, and config tests pass (128 tests)
• Build succeeds with zero warnings
• Manual: run a multi-turn tool loop session and verify distillation fires mid-session (check logs for session_observer_turn_trigger)
• Manual: verify idle-timeout distillation still fires when session goes quiet between turn triggers