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-protocol.ts

@@ -0,0 +1,19 @@
+/**
+ * Agent Protocol Constants (SSOT).
+ *
+ * Shared constants for host ↔ runner communication.
+ * Runners are separate packages and can't import this directly —
+ * they define local copies with comments referencing this file.
+ */
+
+/** Sentinel markers for robust stdout output parsing. */
+export const OUTPUT_START_MARKER = '---EJCLAW_OUTPUT_START---';
+export const OUTPUT_END_MARKER = '---EJCLAW_OUTPUT_END---';
+
+/** Regex to extract [Image: /path/to/file] tags from agent text. */
+export const IMAGE_TAG_RE = /\[Image:\s*(\/[^\]]+)\]/g;
+
+/** IPC polling interval (ms) used by runners to check for follow-up messages. */
+export const IPC_POLL_MS = 500;
+export const IPC_INPUT_SUBDIR = 'input';
+export const IPC_CLOSE_SENTINEL = '_close';