Refactor: move compaction code to src/core/compaction/

- Move compaction.ts to src/core/compaction/compaction.ts
- Extract branch summarization to src/core/compaction/branch-summarization.ts
- Add index.ts to re-export all compaction utilities
- Update all imports across the codebase

packages/coding-agent/test/compaction.test.ts

@@ -12,7 +12,7 @@ import {
 	findCutPoint,
 	getLastAssistantUsage,
 	shouldCompact,
-} from "../src/core/compaction.js";
+} from "../src/core/compaction/index.js";
 import {
 	buildSessionContext,
 	type CompactionEntry,