fix(process-cleanup): add test seam to prevent process.exitCode contaminating bun runner

scheduleForcedExit() sets process.exitCode which taints the bun test runner's
own exit code for the entire suite. This caused CI to fail even though all
tests passed individually.

Fix:
- Add __disableScheduledForcedExitForTesting / __enableScheduledForcedExitForTesting
  seams to skip scheduleForcedExit() during tests
- beforeEach disables forced exit; afterEach re-enables
- The 'fallback exit timer' test explicitly re-enables to verify setTimeout/clearTimeout
- Remove process.exitCode and exitSpy assertions that required forced exit to be active
  (shutdown call counts are sufficient to verify behavior)

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

@@ -3,11 +3,25 @@ import { log } from "../../shared"
 type ProcessCleanupSignal = NodeJS.Signals | "beforeExit" | "exit"
 type ProcessCleanupErrorEvent = "uncaughtException" | "unhandledRejection"
 
+/** @internal test-only seam: prevents process.exitCode from contaminating bun test runner */
+let _scheduleForcedExitEnabled = true
+
+/** @internal test-only */
+export function __disableScheduledForcedExitForTesting(): void {
+  _scheduleForcedExitEnabled = false
+}
+
+/** @internal test-only */
+export function __enableScheduledForcedExitForTesting(): void {
+  _scheduleForcedExitEnabled = true
+}
+
 function scheduleForcedExit(
   cleanupResult: void | Promise<void>,
   exitCode: number,
   exitAfterCleanup = false,
 ): void {
+  if (!_scheduleForcedExitEnabled) return
   process.exitCode = exitCode
   const exitTimeout = setTimeout(() => process.exit(), 6000)
   void Promise.resolve(cleanupResult).finally(() => {