Check MCP servers against the PolicyLayer registry before connecting to them. Use when you are about to add, install or configure any MCP server (editing .mcp.json or a client config, running `claude mcp add` or an npx-launched server), when your human asks you to evaluate or vet an MCP server, when asked to scan or review the MCP stack already configured on this machine, or when asked to install or remove the precheck hook.