Use markdown for migration screen (#8952)

Next steps will be routing this to model info

codex-rs/tui/src/model_migration.rs

@@ -1,4 +1,5 @@
 use crate::key_hint;
+use crate::markdown_render::render_markdown_text_with_width;
 use crate::render::Insets;
 use crate::render::renderable::ColumnRenderable;
 use crate::render::renderable::Renderable;
@@ -34,6 +35,7 @@ pub(crate) struct ModelMigrationCopy {
     pub heading: Vec<Span<'static>>,
     pub content: Vec<Line<'static>>,
     pub can_opt_out: bool,
+    pub markdown: Option<String>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -55,15 +57,30 @@ impl MigrationMenuOption {
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 pub(crate) fn migration_copy_for_models(
     current_model: &str,
     target_model: &str,
     model_link: Option<String>,
     migration_copy: Option<String>,
+    migration_markdown: Option<String>,
     target_display_name: String,
     target_description: Option<String>,
     can_opt_out: bool,
 ) -> ModelMigrationCopy {
+    if let Some(migration_markdown) = migration_markdown {
+        return ModelMigrationCopy {
+            heading: Vec::new(),
+            content: Vec::new(),
+            can_opt_out,
+            markdown: Some(fill_migration_markdown(
+                &migration_markdown,
+                current_model,
+                target_model,
+            )),
+        };
+    }
+
     let heading_text = Span::from(format!(
         "Codex just got an upgrade. Introducing {target_display_name}."
     ))
@@ -113,6 +130,7 @@ pub(crate) fn migration_copy_for_models(
         heading: vec![heading_text],
         content,
         can_opt_out,
+        markdown: None,
     }
 }
 
@@ -237,9 +255,13 @@ impl WidgetRef for &ModelMigrationScreen {
 
         let mut column = ColumnRenderable::new();
         column.push("");
-        column.push(self.heading_line());
-        column.push(Line::from(""));
-        self.render_content(&mut column);
+        if let Some(markdown) = self.copy.markdown.as_ref() {
+            self.render_markdown_content(markdown, area.width, &mut column);
+        } else {
+            column.push(self.heading_line());
+            column.push(Line::from(""));
+            self.render_content(&mut column);
+        }
         if self.copy.can_opt_out {
             self.render_menu(&mut column);
         }
@@ -290,6 +312,21 @@ impl ModelMigrationScreen {
         }
     }
 
+    fn render_markdown_content(
+        &self,
+        markdown: &str,
+        area_width: u16,
+        column: &mut ColumnRenderable,
+    ) {
+        let horizontal_inset = 2;
+        let content_width = area_width.saturating_sub(horizontal_inset);
+        let wrap_width = (content_width > 0).then_some(content_width as usize);
+        let rendered = render_markdown_text_with_width(markdown, wrap_width);
+        for line in rendered.lines {
+            column.push(line.inset(Insets::tlbr(0, horizontal_inset, 0, 0)));
+        }
+    }
+
     fn render_menu(&self, column: &mut ColumnRenderable) {
         column.push(Line::from(""));
         column.push(
@@ -348,6 +385,12 @@ fn is_ctrl_exit_combo(key_event: KeyEvent) -> bool {
         && matches!(key_event.code, KeyCode::Char('c') | KeyCode::Char('d'))
 }
 
+fn fill_migration_markdown(template: &str, current_model: &str, target_model: &str) -> String {
+    template
+        .replace("{model_from}", current_model)
+        .replace("{model_to}", target_model)
+}
+
 #[cfg(test)]
 mod tests {
     use super::ModelMigrationScreen;
@@ -378,6 +421,7 @@ mod tests {
                     "Upgrade to gpt-5.2-codex for the latest and greatest agentic coding model."
                         .to_string(),
                 ),
+                None,
                 "gpt-5.1-codex-max".to_string(),
                 Some("Codex-optimized flagship for deep and fast reasoning.".to_string()),
                 true,
@@ -406,6 +450,7 @@ mod tests {
                 "gpt-5.1",
                 Some("https://www.codex.com/models/gpt-5.1".to_string()),
                 None,
+                None,
                 "gpt-5.1".to_string(),
                 Some("Broad world knowledge with strong general reasoning.".to_string()),
                 false,
@@ -432,6 +477,7 @@ mod tests {
                 "gpt-5.1-codex-max",
                 Some("https://www.codex.com/models/gpt-5.1-codex-max".to_string()),
                 None,
+                None,
                 "gpt-5.1-codex-max".to_string(),
                 Some("Codex-optimized flagship for deep and fast reasoning.".to_string()),
                 false,
@@ -458,6 +504,7 @@ mod tests {
                 "gpt-5.1-codex-mini",
                 Some("https://www.codex.com/models/gpt-5.1-codex-mini".to_string()),
                 None,
+                None,
                 "gpt-5.1-codex-mini".to_string(),
                 Some("Optimized for codex. Cheaper, faster, but less capable.".to_string()),
                 false,
@@ -480,6 +527,7 @@ mod tests {
                 "gpt-new",
                 Some("https://www.codex.com/models/gpt-new".to_string()),
                 None,
+                None,
                 "gpt-new".to_string(),
                 Some("Latest recommended model for better performance.".to_string()),
                 true,
@@ -508,6 +556,7 @@ mod tests {
                 "gpt-new",
                 Some("https://www.codex.com/models/gpt-new".to_string()),
                 None,
+                None,
                 "gpt-new".to_string(),
                 Some("Latest recommended model for better performance.".to_string()),
                 true,