feat(mcp): add mcp_prompt meta-tool and harden prompt display rendering

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).

src/config/mod.rs

@@ -55,7 +55,9 @@ pub use self::skill_policy::SkillPolicy;
 pub use self::skill_registry::SkillRegistry;
 #[cfg(test)]
 pub(crate) use self::tool_scope::test_fixtures;
-pub use self::tool_scope::{McpPromptCompletion, flatten_prompt_messages};
+pub use self::tool_scope::{
+    McpPromptCompletion, flatten_prompt_messages, resolve_prompt_args, sanitize_display_text,
+};
 pub use self::update::run_self_update;
 use crate::client::{
     self, ClientConfig, MessageContentToolCalls, Model, ModelType, OPENAI_COMPATIBLE_PROVIDERS,