fix: consolidate auth error handling and fix start.bat credential check

This commit addresses issues found during review of PRs #12 and #28:

## PR #12 (Auth Error Handling) Fixes

- Create shared auth.py module with centralized AUTH_ERROR_PATTERNS,
  is_auth_error(), and print_auth_error_help() functions
- Fix start.bat to use directory check instead of outdated
  .credentials.json file check (matching start.sh behavior)
- Update process_manager.py to import from shared auth module
- Update start.py to import from shared auth module
- Update documentation comments in autonomous_agent_demo.py and
  client.py to remove references to deprecated .credentials.json

## PR #28 (Feature Management) Improvements

- Add _priority_lock threading lock to feature_mcp.py to prevent
  race conditions when multiple features are created simultaneously
- Apply lock to feature_create, feature_create_bulk, and feature_skip
- Add checkAndSendTimeoutRef cleanup in useAssistantChat.ts to
  prevent memory leaks on component unmount
- Clear currentAssistantMessageRef on response_done

## Code Quality

- All Python files pass ruff linting
- All security tests pass (91/91)
- UI passes ESLint and TypeScript compilation

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

client.py

@@ -92,7 +92,7 @@ def create_client(project_dir: Path, model: str, yolo_mode: bool = False):
        (see security.py for ALLOWED_COMMANDS)
 
     Note: Authentication is handled by start.bat/start.sh before this runs.
-    The Claude SDK auto-detects credentials from ~/.claude/.credentials.json
+    The Claude SDK auto-detects credentials from the Claude CLI configuration
     """
     # Build allowed tools list based on mode
     # In YOLO mode, exclude Playwright tools for faster prototyping