refactor(core): add headless run and auto-restart

- Run without console by default
- Log to file once console hidden
- Recover panics in background goroutines
- Restart agent after unexpected errors
- Use shared instance launch helper

lib/instance/launch_stub.go

@@ -0,0 +1,11 @@
+//go:build !windows
+
+package instance
+
+func Launch(exe string, args []string) error {
+	return nil
+}
+
+func RestartSelf() error {
+	return nil
+}