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- Fix start.bat to use directory check instead of outdated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

start.py

@@ -9,11 +9,11 @@ Supports two paths for new projects:
 """
 
 import os
-import re
 import subprocess
 import sys
 from pathlib import Path
 
+from auth import is_auth_error, print_auth_error_help
 from prompts import (
     get_project_prompts_dir,
     has_project_prompts,
@@ -25,53 +25,6 @@ from registry import (
     register_project,
 )
 
-# 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(output: str) -> bool:
-    """
-    Check if output contains Claude CLI authentication error messages.
-
-    Args:
-        output: Combined stdout/stderr from subprocess
-
-    Returns:
-        True if authentication error detected, False otherwise
-    """
-    if not output:
-        return False
-
-    output_lower = output.lower()
-    for pattern in AUTH_ERROR_PATTERNS:
-        if re.search(pattern, output_lower):
-            return True
-    return False
-
-
-def print_auth_error_help() -> None:
-    """Print helpful message when authentication error is detected."""
-    print("\n" + "=" * 50)
-    print("  Authentication Error Detected")
-    print("=" * 50)
-    print("\nClaude CLI requires authentication to work.")
-    print("\nTo fix this, run:")
-    print("  claude login")
-    print("\nThis will open a browser window to sign in.")
-    print("After logging in, try running this command again.")
-    print("=" * 50 + "\n")
-
 
 def check_spec_exists(project_dir: Path) -> bool:
     """