fix(runtime-fallback): address cubic AI review issues

- Add normalizeFallbackModels helper to centralize string/array normalization (P3)
- Export RuntimeFallbackConfig and FallbackModels types from config/index.ts
- Fix agent detection regex to use word boundaries for sessionID matching
- Improve tests to verify actual fallback switching logic (not just log paths)
- Add SessionCategoryRegistry cleanup in executeSyncTask on completion/error (P2)
- All 24 runtime-fallback tests pass, 115 delegate-task tests pass

src/tools/delegate-task/sync-task.ts

@@ -150,6 +150,7 @@ session_id: ${sessionID}
   } finally {
     if (syncSessionID) {
       subagentSessions.delete(syncSessionID)
+      SessionCategoryRegistry.remove(syncSessionID)
     }
   }
 }