fix(background-agent): reset process.exitCode to 0 between cleanup tests

CI test suite exited 1 despite 0 failing tests because process-cleanup.test.ts
assertions left process.exitCode=1 in place. The afterEach hook only reset to
originalExitCode (which starts undefined), not 0, so Bun picked up exitCode=1
on shutdown and reported the shared batch as failing.

Explicitly set process.exitCode = 0 in beforeEach and afterEach so each test
starts and ends with a clean exit state.

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

@@ -15,10 +15,9 @@ type CleanupManager = {
 
 describe("#given process cleanup registration", () => {
   const registeredManagers: CleanupManager[] = []
-  const originalExitCode = process.exitCode
 
   beforeEach(() => {
-    process.exitCode = originalExitCode
+    process.exitCode = 0
     registeredManagers.length = 0
     _resetForTesting()
   })
@@ -28,7 +27,7 @@ describe("#given process cleanup registration", () => {
       unregisterManagerForCleanup(manager)
     }
 
-    process.exitCode = originalExitCode
+    process.exitCode = 0
     _resetForTesting()
   })