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/instance/mutex_stub.go

@@ -0,0 +1,13 @@
+//go:build !windows
+
+package instance
+
+type Guard struct{}
+
+func Acquire() (*Guard, error) {
+	return &Guard{}, nil
+}
+
+func AlreadyRunning() bool { return false }
+
+func (g *Guard) Close() {}