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/message-runtime.ts

@@ -1,4 +1,5 @@
 import { AgentOutput } from './agent-runner.js';
+import { getErrorMessage } from './utils.js';
 import {
   getMessagesSinceSeq,
   getNewMessagesBySeq,
@@ -119,7 +120,7 @@ export function createMessageRuntime(deps: MessageRuntimeDeps): {
       );
       return true;
     } catch (err) {
-      const errorMessage = err instanceof Error ? err.message : String(err);
+      const errorMessage = getErrorMessage(err);
       markWorkItemDeliveryRetry(item.id, errorMessage);
       logger.warn(
         {