ci: add pytest and ruff linting to CI, fix all lint errors

- Add test job to build.yaml that runs pytest and ruff before building images
- Add standalone test.yaml workflow for PRs
- Add ruff.toml with E/F/I rules configured
- Fix all ruff lint errors: sort imports, remove unused imports, fix re-exports
- Build jobs now depend on test job passing (needs: test)

Closes leeworks-agents/SPARC#18
Closes leeworks-agents/SPARC#19

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SPARC/database.py

@@ -1,14 +1,15 @@
 """Database client for storing and retrieving LLM messages and user authentication."""
 
 import contextlib
-import psycopg2
-from psycopg2.pool import ThreadedConnectionPool
-from psycopg2.extras import RealDictCursor
-from typing import Dict, List, Optional
-from datetime import datetime, timedelta
-import json
 import hashlib
+import json
+from datetime import datetime, timedelta
+from typing import Dict, List, Optional
+
 import bcrypt
+import psycopg2
+from psycopg2.extras import RealDictCursor
+from psycopg2.pool import ThreadedConnectionPool
 
 
 class DatabaseClient: