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/process-cleanup.ts

@@ -11,7 +11,7 @@ function registerProcessSignal(
     handler()
     if (exitAfter) {
       process.exitCode = 0
-      setTimeout(() => process.exit(), 6000)
+      setTimeout(() => process.exit(), 6000).unref()
     }
   }
   process.on(signal, listener)