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