[TUI] add external config migration prompt when start TUI (#17891)

- add a TUI startup migration prompt for external agent config
- support migrating external configs including config, skills, AGENTS.md
and plugins
- gate the prompt behind features.external_migrate (default false)

<img width="1037" height="480" alt="Screenshot 2026-04-14 at 9 29 14 PM"
src="https://github.com/user-attachments/assets/6060849b-03cb-429a-9c13-c7bb46ad2e65"
/>
<img width="713" height="183" alt="Screenshot 2026-04-14 at 9 29 26 PM"
src="https://github.com/user-attachments/assets/d13f177e-d4c4-479c-8736-ef29636081e1"
/>

---------

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/tui/src/app_server_session.rs

@@ -14,6 +14,11 @@ use codex_app_server_protocol::AuthMode;
 use codex_app_server_protocol::ClientRequest;
 use codex_app_server_protocol::ConfigBatchWriteParams;
 use codex_app_server_protocol::ConfigWriteResponse;
+use codex_app_server_protocol::ExternalAgentConfigDetectParams;
+use codex_app_server_protocol::ExternalAgentConfigDetectResponse;
+use codex_app_server_protocol::ExternalAgentConfigImportParams;
+use codex_app_server_protocol::ExternalAgentConfigImportResponse;
+use codex_app_server_protocol::ExternalAgentConfigMigrationItem;
 use codex_app_server_protocol::GetAccountParams;
 use codex_app_server_protocol::GetAccountRateLimitsResponse;
 use codex_app_server_protocol::GetAccountResponse;
@@ -286,6 +291,31 @@ impl AppServerSession {
             .wrap_err("account/read failed during TUI bootstrap")
     }
 
+    pub(crate) async fn external_agent_config_detect(
+        &mut self,
+        params: ExternalAgentConfigDetectParams,
+    ) -> Result<ExternalAgentConfigDetectResponse> {
+        let request_id = self.next_request_id();
+        self.client
+            .request_typed(ClientRequest::ExternalAgentConfigDetect { request_id, params })
+            .await
+            .wrap_err("externalAgentConfig/detect failed during TUI startup")
+    }
+
+    pub(crate) async fn external_agent_config_import(
+        &mut self,
+        migration_items: Vec<ExternalAgentConfigMigrationItem>,
+    ) -> Result<ExternalAgentConfigImportResponse> {
+        let request_id = self.next_request_id();
+        self.client
+            .request_typed(ClientRequest::ExternalAgentConfigImport {
+                request_id,
+                params: ExternalAgentConfigImportParams { migration_items },
+            })
+            .await
+            .wrap_err("externalAgentConfig/import failed during TUI startup")
+    }
+
     pub(crate) async fn next_event(&mut self) -> Option<AppServerEvent> {
         self.client.next_event().await
     }