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/index.ts

@@ -438,6 +438,7 @@ async function main(): Promise<void> {
     writeGroupsSnapshot,
   });
   queue.setProcessMessagesFn(runtime.processGroupMessages);
+  queue.enterRecoveryMode();
   runtime.recoverPendingMessages();
   const restartContext = await announceRestartRecovery(processStartedAtMs);
   for (const candidate of getInterruptedRecoveryCandidates(