Rebrand auth guidance strings: grok login/logout -> kigi login/logout

The headless not-signed-in message still pointed at `grok login` and
XAI_API_KEY; error copy across the shell (re-auth prompts, managed-config
rejection, mcp doctor, trace classifier) and kigi-bin did too. All now
reference `kigi login` / `kigi logout`, and the headless fallback points
at the Moonshot open-platform key (KIGI_MOONSHOT_API_KEY).

crates/codegen/kigi-shell/src/mcp_doctor.rs

@@ -284,7 +284,7 @@ async fn try_discover_managed_servers() -> (ConfigSourceStatus, Vec<DiscoveredSe
 
     let token = match auth_manager.get_valid_token().await {
         Ok(key) => key,
-        Err(_) => return managed_skipped("auth expired — run `grok login`"),
+        Err(_) => return managed_skipped("auth expired — run `kigi login`"),
     };
 
     let proxy_url = crate::agent::config::EndpointsConfig::from_effective_config().proxy_url();