Threshold-driven auto-compaction with strategy=shake auto-continued even when
the shake reclaimed nothing material, and the next agent turn re-triggered the
same shake (which had nothing new to drop on a second pass), spinning forever.
After shake completes, recompute the post-shake context estimate; when it
still exceeds the auto-compact threshold (or shake reclaimed nothing on overflow
recovery), emit a one-shot fallback warning and hand off to the summarization
driven context-full path so progress actually resumes. Idle is exempt — its
60s+ timer self-throttles and cannot dead-loop on its own.
Fixes#2119
- Dropped `summarizeShakeRegions`, the shake-summary prompt, and related types.
- Removed `shake-summary` compaction strategy and `providers.shakeSummaryModel` setting.
- Migrated existing `shake-summary` configs to plain `shake` on load.
- Simplified `/shake` to `elide` and `images` modes only.
roboomp