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-snapshot.ts

@@ -49,5 +49,9 @@ export function writeGroupsSnapshot(
   fs.mkdirSync(groupIpcDir, { recursive: true });
   const visibleGroups = isMain ? groups : [];
   const groupsFile = path.join(groupIpcDir, 'available_groups.json');
-  writeJsonFile(groupsFile, { groups: visibleGroups, lastSync: new Date().toISOString() }, true);
+  writeJsonFile(
+    groupsFile,
+    { groups: visibleGroups, lastSync: new Date().toISOString() },
+    true,
+  );
 }