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/install/relaunch_stub.go

@@ -0,0 +1,21 @@
+//go:build !windows
+
+package install
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func relaunchInstalled(target string) error {
+	cmd := exec.Command(target, os.Args[1:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("relaunch from %s: %w", target, err)
+	}
+	os.Exit(0)
+	return nil
+}