fix: simplify plugin lifecycle - no install/uninstall = just toggle

- Plugins without on_install hook: always 'installed', toggle works directly
- Plugins with on_install: must Install first, then toggle, then Uninstall available
- No data cleanup on Disable (only on Uninstall via on_uninstall hook)
- Old plugins without lifecycle hooks simply don't get install/uninstall UI

internal/core/plugins/manager.go

@@ -354,13 +354,12 @@ type NodeMeta struct {
 
 // Enable activates a plugin by name.
 // If the plugin has on_install hook, it must be installed first.
-// Plugins without on_install hook are always considered "installed".
 func (m *Manager) Enable(name string) error {
 	for i := range m.plugins {
 		if m.plugins[i].Meta.Name == name {
 			p := &m.plugins[i]
 			if p.HasInstall && !p.Installed {
-				return fmt.Errorf("plugin %q must be installed first (use Install)", name)
+				return fmt.Errorf("plugin %q must be installed first", name)
 			}
 			p.Active = true
 			return nil