fix: resolve CI linting errors for Python and ESLint

Python (ruff F401 - unused imports):
- Remove unused DEFAULT_YOLO_MODE import from server/routers/settings.py
- Remove unused AVAILABLE_MODELS import from server/schemas.py

ESLint (missing config for v9):
- Add ui/eslint.config.js with flat config format for ESLint v9
- Configure TypeScript, React Hooks, and React Refresh plugins
- Fix unnecessary regex escapes in AgentThought.tsx
- Remove unused onComplete from useSpecChat.ts dependency array

Additional:
- Add .claude/commands/check-code.md for local CI verification

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/routers/settings.py

@@ -21,7 +21,6 @@ if str(ROOT_DIR) not in sys.path:
 from registry import (
     AVAILABLE_MODELS,
     DEFAULT_MODEL,
-    DEFAULT_YOLO_MODE,
     get_all_settings,
     set_setting,
 )

server/schemas.py

@@ -18,7 +18,7 @@ _root = Path(__file__).parent.parent
 if str(_root) not in sys.path:
     sys.path.insert(0, str(_root))
 
-from registry import AVAILABLE_MODELS, DEFAULT_MODEL, VALID_MODELS
+from registry import DEFAULT_MODEL, VALID_MODELS
 
 # ============================================================================
 # Project Schemas