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/utils.ts

@@ -0,0 +1,69 @@
+/**
+ * Shared utilities (SSOT).
+ *
+ * Small helpers that were previously copy-pasted across 10+ files.
+ */
+
+import fs from 'fs';
+
+// ── Error handling ──────────────────────────────────────────────
+
+/** Extract a human-readable message from an unknown caught value. */
+export function getErrorMessage(err: unknown): string {
+  return err instanceof Error ? err.message : String(err);
+}
+
+// ── JSON file I/O ───────────────────────────────────────────────
+
+/** Read and parse a JSON file. Returns null on any failure. */
+export function readJsonFile<T = unknown>(filePath: string): T | null {
+  try {
+    return JSON.parse(fs.readFileSync(filePath, 'utf-8')) as T;
+  } catch {
+    return null;
+  }
+}
+
+/** Write data as JSON to a file. */
+export function writeJsonFile(
+  filePath: string,
+  data: unknown,
+  pretty = false,
+): void {
+  fs.writeFileSync(
+    filePath,
+    JSON.stringify(data, null, pretty ? 2 : undefined),
+  );
+}
+
+// ── Fetch with timeout ──────────────────────────────────────────
+
+/** Wrapper around fetch() that aborts after timeoutMs. */
+export async function fetchWithTimeout(
+  url: string,
+  init: RequestInit,
+  timeoutMs: number,
+): Promise<Response> {
+  const controller = new AbortController();
+  const timer = setTimeout(() => controller.abort(), timeoutMs);
+  try {
+    return await fetch(url, { ...init, signal: controller.signal });
+  } finally {
+    clearTimeout(timer);
+  }
+}
+
+// ── Time formatting ─────────────────────────────────────────────
+
+/** Format milliseconds as Korean elapsed time (e.g. "1시간 2분 30초"). */
+export function formatElapsedKorean(ms: number): string {
+  const totalSeconds = Math.floor(ms / 1000);
+  const hours = Math.floor(totalSeconds / 3600);
+  const minutes = Math.floor((totalSeconds % 3600) / 60);
+  const seconds = totalSeconds % 60;
+  const parts: string[] = [];
+  if (hours > 0) parts.push(`${hours}시간`);
+  if (minutes > 0) parts.push(`${minutes}분`);
+  parts.push(`${seconds}초`);
+  return parts.join(' ');
+}