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/startup.go

@@ -1,3 +1,5 @@
+//go:build windows
+
 package startup
 
 import (
@@ -67,5 +69,6 @@ func installedCommandLine() (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return `"` + exe + `" -background`, nil
+	// Registry Run requires quotes around a path that may contain spaces.
+	return `"` + exe + `"`, nil
 }