fix(background-agent): mock process.exit in forced-exit tests

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/process-cleanup.test.ts

@@ -89,6 +89,7 @@ describe("#given process cleanup registration", () => {
       const sigintListenersBefore = process.listeners("SIGINT")
       const setTimeoutSpy = spyOn(globalThis, "setTimeout")
       const clearTimeoutSpy = spyOn(globalThis, "clearTimeout")
+      const exitSpy = spyOn(process, "exit").mockImplementation((() => undefined) as never)
       // Re-enable forced exit so we can verify setTimeout/clearTimeout are called
       __enableScheduledForcedExitForTesting()
 
@@ -109,13 +110,42 @@ describe("#given process cleanup registration", () => {
 
         expect(setTimeoutSpy).toHaveBeenCalledTimes(1)
         expect(clearTimeoutSpy).toHaveBeenCalledTimes(1)
+        expect(exitSpy).toHaveBeenCalledWith(0)
       } finally {
+        exitSpy.mockRestore()
         setTimeoutSpy.mockRestore()
         clearTimeoutSpy.mockRestore()
         __disableScheduledForcedExitForTesting()
         process.exitCode = 0
       }
     })
+
+    test("#when cleanup finishes after SIGTERM #then process exits after cleanup", async () => {
+      const sigtermListenersBefore = process.listeners("SIGTERM")
+      const exitSpy = spyOn(process, "exit").mockImplementation((() => undefined) as never)
+      const shutdown = mock(async () => {
+        await Promise.resolve()
+      })
+      const manager = { shutdown }
+      registeredManagers.push(manager)
+      __enableScheduledForcedExitForTesting()
+
+      try {
+        registerManagerForCleanup(manager)
+
+        const sigtermListener = getNewListener("SIGTERM", sigtermListenersBefore)
+
+        sigtermListener()
+        await flushMicrotasks()
+
+        expect(shutdown).toHaveBeenCalledTimes(1)
+        expect(exitSpy).toHaveBeenCalledWith(0)
+      } finally {
+        exitSpy.mockRestore()
+        __disableScheduledForcedExitForTesting()
+        process.exitCode = 0
+      }
+    })
   })
 
   describe("#given multiple cleanup managers", () => {