fix: add PyYAML as core dependency for workflow preset management

workflow_tools.py imports yaml at module level, but PyYAML was not
declared in requirements.txt or pyproject.toml core dependencies.
This caused ModuleNotFoundError when tools/__init__.py was loaded,
silently breaking server.py's try block and leaving list_tools
undefined -- causing all test_mcp_server.py tests to fail in CI.

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

pyproject.toml

@@ -63,6 +63,7 @@ dependencies = [
     "networkx>=3.0",
     "tomli>=2.0.0; python_version < '3.11'",  # TOML parser for version reading
     "schedule>=1.2.0",  # Required for sync monitoring
+    "PyYAML>=6.0",  # Required for workflow preset management
 ]
 
 [project.optional-dependencies]