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/status-dashboard.ts

@@ -2,6 +2,7 @@ import fs from 'fs';
 import path from 'path';
 
 import { CACHE_DIR } from './config.js';
+import { writeJsonFile } from './utils.js';
 import type { GroupStatus } from './group-queue.js';
 import type { AgentType } from './types.js';
 
@@ -32,7 +33,7 @@ export function writeStatusSnapshot(snapshot: StatusSnapshot): void {
     `${snapshot.agentType}.json`,
   );
   const tempPath = `${targetPath}.tmp`;
-  fs.writeFileSync(tempPath, JSON.stringify(snapshot, null, 2));
+  writeJsonFile(tempPath, snapshot, true);
   fs.renameSync(tempPath, targetPath);
 }