refactor: extract shared utilities, protocol constants, and retry loop

Phase 3: provider-retry.ts — shared Claude rotation loop (SSOT)
  - retryClaudeWithRotation extracted from message-agent-executor + task-scheduler
  - ~200 lines of duplicated retry logic removed

Phase 4: types.ts — AgentOutputPhase + VisiblePhase unified
Phase 5: types.ts — AgentConfig extended with claudeThinking/claudeThinkingBudget

Utilities (utils.ts):
  - getErrorMessage: 14 occurrences of instanceof Error pattern → 1 function
  - readJsonFile/writeJsonFile: 19 occurrences of JSON+fs pattern → 2 functions
  - fetchWithTimeout: 3 occurrences of AbortController pattern → 1 function
  - formatElapsedKorean: deduplicated from task-watch-status + message-turn-controller

Protocol (agent-protocol.ts):
  - OUTPUT_START/END_MARKER centralized (runners keep local copies with SSOT reference)
  - IMAGE_TAG_RE, IPC constants documented

Runner: show "대화 요약 중..." progress message during auto-compact

Net: -137 lines, 354/354 tests passing

src/group-queue-ipc.ts

@@ -1,6 +1,8 @@
 import fs from 'fs';
 import path from 'path';
 
+import { writeJsonFile } from './utils.js';
+
 function resolveInputDir(ipcDir: string): string {
   return path.join(ipcDir, 'input');
 }
@@ -11,7 +13,7 @@ export function queueFollowUpMessage(ipcDir: string, text: string): string {
   const filename = `${Date.now()}-${Math.random().toString(36).slice(2, 6)}.json`;
   const filepath = path.join(inputDir, filename);
   const tempPath = `${filepath}.tmp`;
-  fs.writeFileSync(tempPath, JSON.stringify({ type: 'message', text }));
+  writeJsonFile(tempPath, { type: 'message', text });
   fs.renameSync(tempPath, filepath);
   return filename;
 }