feat: deprecation warning (#5825)

<img width="955" height="311" alt="Screenshot 2025-10-28 at 14 26 25"
src="https://github.com/user-attachments/assets/99729b3d-3bc9-4503-aab3-8dc919220ab4"
/>

codex-rs/tui/src/history_cell.rs

@@ -1005,6 +1005,38 @@ pub(crate) fn new_warning_event(message: String) -> PlainHistoryCell {
     }
 }
 
+#[derive(Debug)]
+pub(crate) struct DeprecationNoticeCell {
+    summary: String,
+    details: Option<String>,
+}
+
+pub(crate) fn new_deprecation_notice(
+    summary: String,
+    details: Option<String>,
+) -> DeprecationNoticeCell {
+    DeprecationNoticeCell { summary, details }
+}
+
+impl HistoryCell for DeprecationNoticeCell {
+    fn display_lines(&self, width: u16) -> Vec<Line<'static>> {
+        let mut lines: Vec<Line<'static>> = Vec::new();
+        lines.push(vec!["⚠ ".red().bold(), self.summary.clone().red()].into());
+
+        let wrap_width = width.saturating_sub(4).max(1) as usize;
+
+        if let Some(details) = &self.details {
+            let line = textwrap::wrap(details, wrap_width)
+                .into_iter()
+                .map(|s| s.to_string().dim().into())
+                .collect::<Vec<_>>();
+            lines.extend(line);
+        }
+
+        lines
+    }
+}
+
 /// Render a summary of configured MCP servers from the current `Config`.
 pub(crate) fn empty_mcp_output() -> PlainHistoryCell {
     let lines: Vec<Line<'static>> = vec![
@@ -2259,4 +2291,21 @@ mod tests {
         let rendered_transcript = render_transcript(cell.as_ref());
         assert_eq!(rendered_transcript, vec!["• We should fix the bug next."]);
     }
+
+    #[test]
+    fn deprecation_notice_renders_summary_with_details() {
+        let cell = new_deprecation_notice(
+            "Feature flag `foo`".to_string(),
+            Some("Use flag `bar` instead.".to_string()),
+        );
+        let lines = cell.display_lines(80);
+        let rendered = render_lines(&lines);
+        assert_eq!(
+            rendered,
+            vec![
+                "⚠ Feature flag `foo`".to_string(),
+                "Use flag `bar` instead.".to_string(),
+            ]
+        );
+    }
 }