feat(cli)!: rename --prompt to --temp-role

Completes the .prompt/.temp-role split: --prompt set an ad-hoc system
role, which is what .temp-role now means everywhere. The --prompt name
is left unbound so a future one-shot MCP prompt flag can take it with
properly designed non-interactive semantics. use_prompt follows the
rename as use_temp_role.

BREAKING CHANGE: invocations using --prompt <text> must switch to
--temp-role <text>; clap rejects the old flag loudly.

src/repl/mod.rs

@@ -819,7 +819,7 @@ pub async fn run_repl_command(
             ".temp-role" => match args {
                 Some(text) => {
                     let app = Arc::clone(&ctx.app.config);
-                    ctx.use_prompt(app.as_ref(), text)?;
+                    ctx.use_temp_role(app.as_ref(), text)?;
                 }
                 None => println!("Usage: .temp-role <text>..."),
             },