feat: route staged scene skills through runtime

Add registry-driven scene routing and multi-root skill loading so fault-details and 95598 scene skills can be triggered from natural language while still running through the browser-backed runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/deepseek_provider_test.rs

@@ -21,7 +21,7 @@ fn deepseek_settings_load_defaults_from_env() {
     assert_eq!(settings.api_key, "test-key");
     assert_eq!(settings.base_url, "https://api.deepseek.com");
     assert_eq!(settings.model, "deepseek-chat");
-    assert_eq!(settings.skills_dir, None);
+    assert!(settings.skills_dir.is_empty());
 }
 
 #[test]
@@ -30,7 +30,7 @@ fn deepseek_request_shape_matches_openai_compatible_chat_format() {
         api_key: "test-key".to_string(),
         base_url: "https://api.deepseek.com".to_string(),
         model: "deepseek-chat".to_string(),
-        skills_dir: None,
+        skills_dir: Vec::new(),
     });
     let messages = vec![
         ChatMessage {