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/src/core/compaction/index.ts

@@ -0,0 +1,6 @@
+/**
+ * Compaction and summarization utilities.
+ */
+
+export * from "./branch-summarization.js";
+export * from "./compaction.js";