Idle-time consolidation: run compaction during quiet periods

Compaction is currently invoked explicitly. Running it during idle periods is unusually low-risk in this architecture because every proposed change is validated before Palinode applies it and git records the result. Open questions for discussion: idle detection, batching, and how ambient runs surface their commits for review.

Start by tracing where compaction is currently invoked. Then investigate the three open questions in the issue: idle detection, batching, and how ambient runs expose their commits for review. Done requires an agreed design and implementation for running validated compaction during quiet periods with reviewable commit history.

Written by the indexing model from the issue text.