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/claude-usage.ts

@@ -55,7 +55,8 @@ let diskCacheLoaded = false;
 function loadUsageDiskCache(): void {
   if (diskCacheLoaded) return;
   diskCacheLoaded = true;
-  usageDiskCache = readJsonFile<Record<string, UsageCacheEntry>>(USAGE_CACHE_FILE) ?? {};
+  usageDiskCache =
+    readJsonFile<Record<string, UsageCacheEntry>>(USAGE_CACHE_FILE) ?? {};
 }
 
 function saveUsageDiskCache(): void {