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- Fix start.bat to use directory check instead of outdated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>

server/services/process_manager.py

@@ -18,6 +18,11 @@ from typing import Awaitable, Callable, Literal, Set
 
 import psutil
 
+# Add parent directory to path for shared module imports
+sys.path.insert(0, str(Path(__file__).parent.parent.parent))
+from auth import AUTH_ERROR_HELP_SERVER as AUTH_ERROR_HELP  # noqa: E402
+from auth import is_auth_error
+
 logger = logging.getLogger(__name__)
 
 # Patterns for sensitive data that should be redacted from output
@@ -36,47 +41,6 @@ SENSITIVE_PATTERNS = [
     r'aws[_-]?secret[=:][^\s]+',
 ]
 
-# Patterns that indicate Claude CLI authentication errors
-AUTH_ERROR_PATTERNS = [
-    r"not\s+logged\s+in",
-    r"not\s+authenticated",
-    r"authentication\s+(failed|required|error)",
-    r"login\s+required",
-    r"please\s+(run\s+)?['\"]?claude\s+login",
-    r"unauthorized",
-    r"invalid\s+(token|credential|api.?key)",
-    r"expired\s+(token|session|credential)",
-    r"could\s+not\s+authenticate",
-    r"sign\s+in\s+(to|required)",
-]
-
-
-def is_auth_error(text: str) -> bool:
-    """Check if text contains Claude CLI authentication error messages."""
-    if not text:
-        return False
-    text_lower = text.lower()
-    for pattern in AUTH_ERROR_PATTERNS:
-        if re.search(pattern, text_lower):
-            return True
-    return False
-
-
-AUTH_ERROR_HELP = """
-================================================================================
-  AUTHENTICATION ERROR DETECTED
-================================================================================
-
-Claude CLI requires authentication to work.
-
-To fix this, run:
-  claude login
-
-This will open a browser window to sign in.
-After logging in, try starting the agent again.
-================================================================================
-"""
-
 
 def sanitize_output(line: str) -> str:
     """Remove sensitive information from output lines."""