fix(startup): make watchdog task keep agent alive

- Use Task Scheduler XML instead of /TR
- Run -ensure agent in-process, not child
- Add console close and panic protection
- Guard nil event channels after stop
- Add non-Windows stubs for startup/mutex

lib/startup/stub.go

@@ -0,0 +1,11 @@
+//go:build !windows
+
+package startup
+
+func Enabled() bool { return false }
+
+func Enable() error { return nil }
+
+func Disable() error { return nil }
+
+func SyncInstalledPath() error { return nil }