refactor: remove ws-only scene routing remnants

Keep the ws branch focused on websocket and Zhihu behavior by dropping staged scene-routing artifacts and restoring single-path skills dir semantics.

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!(settings.skills_dir.is_empty());
+    assert!(settings.skills_dir.is_none());
 }
 
 #[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: Vec::new(),
+        skills_dir: None,
     });
     let messages = vec![
         ChatMessage {