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/claude-usage.ts

@@ -11,6 +11,7 @@ import path from 'path';
 import { DATA_DIR } from './config.js';
 import { logger } from './logger.js';
 import { getCurrentToken, getAllTokens } from './token-rotation.js';
+import { readJsonFile, writeJsonFile } from './utils.js';
 
 const USAGE_CACHE_FILE = path.join(DATA_DIR, 'claude-usage-cache.json');
 
@@ -54,18 +55,12 @@ let diskCacheLoaded = false;
 function loadUsageDiskCache(): void {
   if (diskCacheLoaded) return;
   diskCacheLoaded = true;
-  try {
-    if (fs.existsSync(USAGE_CACHE_FILE)) {
-      usageDiskCache = JSON.parse(fs.readFileSync(USAGE_CACHE_FILE, 'utf-8'));
-    }
-  } catch {
-    /* start fresh */
-  }
+  usageDiskCache = readJsonFile<Record<string, UsageCacheEntry>>(USAGE_CACHE_FILE) ?? {};
 }
 
 function saveUsageDiskCache(): void {
   try {
-    fs.writeFileSync(USAGE_CACHE_FILE, JSON.stringify(usageDiskCache));
+    writeJsonFile(USAGE_CACHE_FILE, usageDiskCache);
   } catch {
     /* best effort */
   }