Emit an mcp_prompt_<server> declaration for servers advertising the
prompts capability, execute prompts via McpRuntime::prompt on both tool
dispatch chains, and return the flattened prompt text as the tool
result. Sanitize server-controlled prompt names, descriptions, and
argument names before terminal rendering, and attribute the .prompt
argument inquire label to its server and prompt.
Per plans/mcp-resources-prompts-design.md §5.2 (T7).