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/config/mod.rs

@@ -55,6 +55,7 @@ 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::update::run_self_update;
 use crate::client::{
     self, ClientConfig, MessageContentToolCalls, Model, ModelType, OPENAI_COMPATIBLE_PROVIDERS,