refactor: add recovery mode + shared JSON/error/fetch utilities

- GroupQueue recovery mode: limit to 3 concurrent agents for 60s after restart
  to prevent API rate-limit storms (configurable via env vars)
- getErrorMessage: replace 14 occurrences of instanceof Error pattern
- readJsonFile/writeJsonFile: replace 19 occurrences of JSON+fs pattern
- fetchWithTimeout: replace 3 occurrences of AbortController pattern

354/354 tests passing

src/agent-runner.test.ts

@@ -4,10 +4,7 @@ import { PassThrough } from 'stream';
 import fs from 'fs';
 import { spawn } from 'child_process';
 
-import {
-  OUTPUT_START_MARKER,
-  OUTPUT_END_MARKER,
-} from './agent-protocol.js';
+import { OUTPUT_START_MARKER, OUTPUT_END_MARKER } from './agent-protocol.js';
 
 // Mock config
 vi.mock('./config.js', () => ({