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/args.go

@@ -0,0 +1,21 @@
+package instance
+
+import "strings"
+
+func filterArgs(args []string) []string {
+	out := make([]string, 0, len(args))
+	for _, arg := range args {
+		name := strings.TrimLeft(arg, "-/")
+		lower := strings.ToLower(name)
+		switch {
+		case lower == "background" || strings.HasPrefix(lower, "background="):
+			continue
+		case lower == "foreground" || strings.HasPrefix(lower, "foreground="):
+			continue
+		case lower == "ensure" || strings.HasPrefix(lower, "ensure="):
+			continue
+		}
+		out = append(out, arg)
+	}
+	return out
+}