feat(cli/demo): add --demo developer message injection

code-rs/exec/src/cli.rs

@@ -76,6 +76,11 @@ pub struct Cli {
     #[clap(skip)]
     pub config_overrides: CliConfigOverrides,
 
+    /// Optional developer-role message to prepend to every turn for demos.
+    /// Set by the multitool root CLI via `--demo`.
+    #[clap(skip)]
+    pub demo_developer_message: Option<String>,
+
     /// Specifies color settings for use in the output.
     #[arg(long = "color", value_enum, default_value_t = Color::Auto)]
     pub color: Color,

code-rs/exec/src/lib.rs

@@ -276,6 +276,7 @@ pub async fn run_main(cli: Cli, code_linux_sandbox_exe: Option<PathBuf>) -> anyh
     };
 
     let mut config = Config::load_with_cli_overrides(cli_kv_overrides, overrides)?;
+    config.demo_developer_message = cli.demo_developer_message.clone();
     let slash_context = SlashContext {
         agents: &config.agents,
         subagent_commands: &config.subagent_commands,