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/agent/agent.go

@@ -21,17 +21,19 @@ import (
 )
 
 type Agent struct {
-	addr      string
-	root      string
-	guard     *instance.Guard
-	server    *http.Server
-	startedAt time.Time
+	addr            string
+	root            string
+	guard           *instance.Guard
+	server          *http.Server
+	startedAt       time.Time
+	stopOnInterrupt bool
 }
 
 // Config controls agent startup. Zero values use environment defaults.
 type Config struct {
-	ListenAddr string
-	Parallel   bool
+	ListenAddr      string
+	Parallel        bool
+	StopOnInterrupt bool
 }
 
 func New(cfg Config) (*Agent, error) {
@@ -40,8 +42,9 @@ func New(cfg Config) (*Agent, error) {
 		addr = config.EnvOr("AGENT_ADDR", config.DefaultAddr)
 	}
 	a := &Agent{
-		addr:      addr,
-		startedAt: time.Now(),
+		addr:            addr,
+		startedAt:       time.Now(),
+		stopOnInterrupt: cfg.StopOnInterrupt,
 	}
 	if root := strings.TrimSpace(os.Getenv("AGENT_FILE_ROOT")); root != "" {
 		resolved, err := files.CanonicalExistingPath(root)
@@ -116,14 +119,16 @@ func (a *Agent) Serve() error {
 
 	helpers.Log.Printf("win64_mp %s listening on http://%s", config.Version, a.addr)
 
-	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
-	defer stop()
-	helpers.Go("shutdown", func() {
-		<-ctx.Done()
-		shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-		defer cancel()
-		_ = a.server.Shutdown(shutdownCtx)
-	})
+	if a.stopOnInterrupt {
+		ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
+		defer stop()
+		helpers.Go("shutdown", func() {
+			<-ctx.Done()
+			shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+			_ = a.server.Shutdown(shutdownCtx)
+		})
+	}
 
 	if err := a.server.Serve(listener); err != nil && !errors.Is(err, http.ErrServerClosed) {
 		return fmt.Errorf("serve: %w", err)