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>

.claude/commands/check-code.md

@@ -0,0 +1,32 @@
+---
+description:
+---
+
+Run the following commands and ensure the code is clean.
+
+From project root:
+
+# Python linting
+
+ruff check .
+
+# Security tests
+
+python test_security.py
+
+From ui/ directory:
+cd ui
+
+# ESLint (will fail until we add the config)
+
+npm run lint
+
+# TypeScript check + build
+
+npm run build
+
+One-liner to run everything:
+ruff check . && python test_security.py && cd ui && npm run lint && npm run build
+
+Or if you want to see all failures at once (doesn't stop on first error):
+ruff check .; python test_security.py; cd ui && npm run lint; npm run build