feat(repl): add .prompt command with live staged tab-completion

Implements plans/mcp-resources-prompts-design.md §5.1/§5.4 (T6):

- .prompt <server> <name> [key=value ...] fetches an MCP prompt and
  submits the result as chat input via Input::from_str + ask(), never
  through REPL line parsing; GetPromptResult messages are flattened
  into one user-role block with unconditional [user]/[assistant] labels
- missing required prompt arguments are collected interactively
- .list prompts renders server/name/description/args via the unified
  catalog (CatalogItem gains an arguments field), degrading per server
- staged live tab-completion: enabled+running+prompts-capable servers
  (no RPC), then live prompt names, then key= argument suggestions with
  (required) markers; 2s timeout per RPC, all errors degrade to silent
  empty suggestions, ctx read guard dropped before blocking
- the enabled-server alias expansion is factored into a shared helper
  used by both tool-scope rebuild and completion
- BREAKING: the former .prompt <text> temp-role builtin is renamed to
  .temp-role <text> (behavior preserved); .prompt now belongs to MCP
  prompts, and a user macro named prompt or temp-role is shadowed

src/mcp/mod.rs

@@ -16,7 +16,7 @@ use futures_util::{StreamExt, TryStreamExt, stream};
 use http::{HeaderName, HeaderValue};
 use indexmap::IndexMap;
 use indoc::formatdoc;
-use rmcp::model::ServerCapabilities;
+use rmcp::model::{PromptArgument, ServerCapabilities};
 use rmcp::service::RunningService;
 use rmcp::transport::StreamableHttpClientTransport;
 use rmcp::transport::TokioChildProcess;
@@ -123,6 +123,8 @@ pub struct CatalogItem {
     pub mime_type: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub size: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub arguments: Option<Vec<PromptArgument>>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]