Extract shared compaction/branch-summarization utils

- New utils.ts with shared functions:
  - FileOperations type and createFileOps()
  - extractFileOpsFromMessage()
  - computeFileLists()
  - formatFileOperations()
  - serializeConversation()
  - SUMMARIZATION_SYSTEM_PROMPT

- branch-summarization.ts now uses:
  - Serialization approach (conversation as text, not LLM messages)
  - completeSimple with system prompt
  - Shared utility functions

packages/coding-agent/src/core/compaction/index.ts

@@ -4,3 +4,4 @@
 
 export * from "./branch-summarization.js";
 export * from "./compaction.js";
+export * from "./utils.js";