fix: address Oracle + Cubic review feedback for background-agent refactoring

- Revert getMessageDir to original join(MESSAGE_STORAGE, sessionID) behavior
- Fix dead subagentSessions.delete by capturing previousSessionID before tryFallbackRetry
- Add .unref() to process cleanup setTimeout to prevent 6s hang on Ctrl-C
- Add missing isUnstableAgent to fallback retry input mapping
- Fix process-cleanup tests to use exit listener instead of SIGINT at index 0
- Swap test filenames in compaction-aware-message-resolver to exercise skip logic correctly

src/features/background-agent/fallback-retry-handler.ts

@@ -117,6 +117,7 @@ export function tryFallbackRetry(args: {
     model: task.model,
     fallbackChain: task.fallbackChain,
     category: task.category,
+    isUnstableAgent: task.isUnstableAgent,
   }
   queue.push({ task, input: retryInput })
   queuesByKey.set(key, queue)