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

runners/codex-runner/src/index.ts

@@ -50,6 +50,7 @@ const IPC_INPUT_DIR = path.join(IPC_DIR, 'input');
 const IPC_INPUT_CLOSE_SENTINEL = path.join(IPC_INPUT_DIR, '_close');
 const IPC_POLL_MS = 500;
 
+/** SSOT: src/agent-protocol.ts — keep in sync */
 const OUTPUT_START_MARKER = '---EJCLAW_OUTPUT_START---';
 const OUTPUT_END_MARKER = '---EJCLAW_OUTPUT_END---';
 
@@ -134,6 +135,7 @@ function drainIpcInput(): string[] {
 }
 
 function extractImagePaths(text: string): { cleanText: string; imagePaths: string[] } {
+  /** SSOT: src/agent-protocol.ts — keep in sync */
   const imagePattern = /\[Image:\s*(\/[^\]]+)\]/g;
   const imagePaths: string[] = [];
   let match: RegExpExecArray | null;