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/db.ts

@@ -18,6 +18,7 @@ import {
   ScheduledTask,
   TaskRunLog,
 } from './types.js';
+import { readJsonFile } from './utils.js';
 
 let db: Database.Database;
 
@@ -1300,14 +1301,14 @@ export function isPairedRoomJid(jid: string): boolean {
 function migrateJsonState(): void {
   const migrateFile = (filename: string) => {
     const filePath = path.join(DATA_DIR, filename);
-    if (!fs.existsSync(filePath)) return null;
+    const data = readJsonFile(filePath);
+    if (data === null) return null;
     try {
-      const data = JSON.parse(fs.readFileSync(filePath, 'utf-8'));
       fs.renameSync(filePath, `${filePath}.migrated`);
-      return data;
     } catch {
-      return null;
+      /* best effort */
     }
+    return data;
   };
 
   // Migrate router_state.json