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/index.ts

@@ -12,6 +12,7 @@ export {
 export { type ApiKeyCredential, type AuthCredential, AuthStorage, type OAuthCredential } from "./core/auth-storage.js";
 // Compaction
 export {
+	type BranchSummaryResult,
 	type CompactionResult,
 	type CutPointResult,
 	calculateContextTokens,
@@ -20,10 +21,11 @@ export {
 	estimateTokens,
 	findCutPoint,
 	findTurnStartIndex,
+	generateBranchSummary,
 	generateSummary,
 	getLastAssistantUsage,
 	shouldCompact,
-} from "./core/compaction.js";
+} from "./core/compaction/index.js";
 // Custom tools
 export type {
 	AgentToolUpdateCallback,