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- Fix start.bat to use directory check instead of outdated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

auth.py

@@ -0,0 +1,83 @@
+"""
+Authentication Error Detection
+==============================
+
+Shared utilities for detecting Claude CLI authentication errors.
+Used by both CLI (start.py) and server (process_manager.py) to provide
+consistent error detection and messaging.
+"""
+
+import re
+
+# Patterns that indicate authentication errors from Claude CLI
+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.
+
+    Uses case-insensitive pattern matching against known error messages.
+
+    Args:
+        text: Output text to check
+
+    Returns:
+        True if any auth error pattern matches, False otherwise
+    """
+    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
+
+
+# CLI-style help message (for terminal output)
+AUTH_ERROR_HELP_CLI = """
+==================================================
+  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 running this command again.
+==================================================
+"""
+
+# Server-style help message (for WebSocket streaming)
+AUTH_ERROR_HELP_SERVER = """
+================================================================================
+  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 print_auth_error_help() -> None:
+    """Print helpful message when authentication error is detected (CLI version)."""
+    print(AUTH_ERROR_HELP_CLI)