[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/features/src/lib.rs

@@ -156,6 +156,8 @@ pub enum Feature {
     ToolSuggest,
     /// Enable plugins.
     Plugins,
+    /// Show the startup prompt for migrating external agent config into Codex.
+    ExternalMigration,
     /// Allow the model to invoke the built-in image generation tool.
     ImageGeneration,
     /// Allow prompting and installing missing MCP dependencies.
@@ -844,6 +846,16 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::ExternalMigration,
+        key: "external_migration",
+        stage: Stage::Experimental {
+            name: "External migration",
+            menu_description: "Show a startup prompt when Codex detects migratable external agent config for this machine or project.",
+            announcement: "",
+        },
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ImageGeneration,
         key: "image_generation",