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.test.ts

@@ -64,7 +64,9 @@ describe("process-cleanup", () => {
 
       registerManagerForCleanup(manager)
 
-      const [, listener] = processOnCalls[0]
+      const exitEntry = processOnCalls.find(([signal]) => signal === "exit")
+      expect(exitEntry).toBeDefined()
+      const [, listener] = exitEntry!
       listener()
 
       expect(mockShutdown).toHaveBeenCalled()
@@ -83,7 +85,9 @@ describe("process-cleanup", () => {
       registerManagerForCleanup(manager2)
       registerManagerForCleanup(manager3)
 
-      const [, listener] = processOnCalls[0]
+      const exitEntry = processOnCalls.find(([signal]) => signal === "exit")
+      expect(exitEntry).toBeDefined()
+      const [, listener] = exitEntry!
       listener()
 
       expect(shutdown1).toHaveBeenCalledTimes(1)
@@ -144,7 +148,9 @@ describe("process-cleanup", () => {
       registerManagerForCleanup(manager1)
       registerManagerForCleanup(manager2)
 
-      const [, listener] = processOnCalls[0]
+      const exitEntry = processOnCalls.find(([signal]) => signal === "exit")
+      expect(exitEntry).toBeDefined()
+      const [, listener] = exitEntry!
       unregisterManagerForCleanup(manager2)
 
       listener()