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>

ui/src/components/AgentThought.tsx

@@ -24,7 +24,7 @@ function isAgentThought(line: string): boolean {
   if (/^Output:/.test(trimmed)) return false
 
   // Skip JSON and very short lines
-  if (/^[\[\{]/.test(trimmed)) return false
+  if (/^[[{]/.test(trimmed)) return false
   if (trimmed.length < 15) return false
 
   // Skip lines that are just paths or technical output

ui/src/hooks/useSpecChat.ts

@@ -33,7 +33,7 @@ function generateId(): string {
 
 export function useSpecChat({
   projectName,
-  onComplete,
+  // onComplete intentionally not used - user clicks "Continue to Project" button instead
   onError,
 }: UseSpecChatOptions): UseSpecChatReturn {
   const [messages, setMessages] = useState<ChatMessage[]>([])
@@ -346,7 +346,7 @@ export function useSpecChat({
         console.error('Failed to parse WebSocket message:', e)
       }
     }
-  }, [projectName, onComplete, onError])
+  }, [projectName, onError])
 
   const start = useCallback(() => {
     connect()