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/agent-runner.test.ts

@@ -4,9 +4,10 @@ import { PassThrough } from 'stream';
 import fs from 'fs';
 import { spawn } from 'child_process';
 
-// Sentinel markers must match agent-runner.ts
-const OUTPUT_START_MARKER = '---EJCLAW_OUTPUT_START---';
-const OUTPUT_END_MARKER = '---EJCLAW_OUTPUT_END---';
+import {
+  OUTPUT_START_MARKER,
+  OUTPUT_END_MARKER,
+} from './agent-protocol.js';
 
 // Mock config
 vi.mock('./config.js', () => ({