[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/config/src/types.rs

@@ -592,7 +592,23 @@ const fn default_true() -> bool {
 /// Settings for notices we display to users via the tui and app-server clients
 /// (primarily the Codex IDE extension). NOTE: these are different from
 /// notifications - notices are warnings, NUX screens, acknowledgements, etc.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, Default, JsonSchema)]
+#[schemars(deny_unknown_fields)]
+pub struct ExternalConfigMigrationPrompts {
+    /// Tracks whether home-level external config migration prompts are hidden.
+    pub home: Option<bool>,
+    /// Tracks the last time the home-level external config migration prompt was shown.
+    pub home_last_prompted_at: Option<i64>,
+    /// Tracks which project paths have opted out of external config migration prompts.
+    #[serde(default)]
+    pub projects: BTreeMap<String, bool>,
+    /// Tracks the last time a project-level external config migration prompt was shown.
+    #[serde(default)]
+    pub project_last_prompted_at: BTreeMap<String, i64>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema)]
+#[schemars(deny_unknown_fields)]
 pub struct Notice {
     /// Tracks whether the user has acknowledged the full access warning prompt.
     pub hide_full_access_warning: Option<bool>,
@@ -608,6 +624,9 @@ pub struct Notice {
     /// Tracks acknowledged model migrations as old->new model slug mappings.
     #[serde(default)]
     pub model_migrations: BTreeMap<String, String>,
+    /// Tracks scopes where external config migration prompts should be suppressed.
+    #[serde(default)]
+    pub external_config_migration_prompts: ExternalConfigMigrationPrompts,
 }
 
 pub use crate::skills_config::BundledSkillsConfig;