Add Markdown conversation export to the TUI (#37358)

## What changed

- Add `/export` with clipboard and file destinations, including a default filename prompt and support for an explicit path argument.
- Export the complete conversation history as structured Markdown, preserving user and assistant messages, plans, reasoning, activity, image labels, file changes, and MCP tool details while honoring reasoning visibility.
- Fall back to legacy history loading when pagination is unavailable, and use the visible transcript for ephemeral sessions.
- Resolve relative and home-relative paths without overwriting an existing file, and report export or copy results in the conversation.

## Testing

- Cover Markdown rendering, hidden review prompts, activity details, complete paginated and legacy history loading, destination prompts, status messages, path resolution, and overwrite protection.

GitOrigin-RevId: c2815fc2430d7a555da75d7038cc1ac08157b82e

codex-rs/tui/src/chatwidget/transcript_export.rs

@@ -0,0 +1,74 @@
+//! Destination and filename prompts for on-demand transcript exports.
+
+use super::*;
+use crate::app_event::TranscriptExportDestination;
+
+impl ChatWidget {
+    pub(crate) fn copy_transcript_to_clipboard(&mut self, markdown: &str) {
+        match crate::clipboard_copy::copy_to_clipboard(markdown) {
+            Ok(lease) => {
+                self.clipboard_lease = lease;
+                self.add_info_message(
+                    "Copied conversation to clipboard".to_string(),
+                    /*hint*/ None,
+                );
+            }
+            Err(error) => self.add_error_message(format!("Copy failed: {error}")),
+        }
+    }
+
+    pub(super) fn show_transcript_export_popup(&mut self) {
+        self.show_selection_view(SelectionViewParams {
+            title: Some("Export conversation".to_string()),
+            subtitle: Some("Save the complete conversation as Markdown".to_string()),
+            footer_hint: Some(standard_popup_hint_line()),
+            items: vec![
+                SelectionItem {
+                    name: "Copy to clipboard".to_string(),
+                    description: Some("Copy the complete Markdown transcript".to_string()),
+                    is_disabled: cfg!(target_os = "android"),
+                    actions: vec![Box::new(|tx| {
+                        tx.send(AppEvent::ExportTranscript {
+                            destination: TranscriptExportDestination::Clipboard,
+                        });
+                    })],
+                    dismiss_on_select: true,
+                    ..Default::default()
+                },
+                SelectionItem {
+                    name: "Save to file".to_string(),
+                    description: Some("Choose a Markdown filename".to_string()),
+                    actions: vec![Box::new(|tx| {
+                        tx.send(AppEvent::OpenTranscriptExportFilePrompt);
+                    })],
+                    dismiss_on_select: true,
+                    ..Default::default()
+                },
+            ],
+            ..Default::default()
+        });
+        self.defer_input_until_settings_applied();
+        self.request_redraw();
+    }
+
+    pub(crate) fn show_transcript_export_file_prompt(&mut self) {
+        let tx = self.app_event_tx.clone();
+        let filename = self.thread_id().map_or_else(
+            || "codex-session.md".to_string(),
+            |thread_id| format!("codex-session-{thread_id}.md"),
+        );
+        let view = CustomPromptView::new(
+            "Save conversation".to_string(),
+            "Type a filename and press Enter".to_string(),
+            filename,
+            /*context_label*/ None,
+            Box::new(move |filename| {
+                tx.send(AppEvent::ExportTranscript {
+                    destination: TranscriptExportDestination::File(PathBuf::from(filename)),
+                });
+            }),
+        );
+        self.bottom_pane.show_view(Box::new(view));
+        self.request_redraw();
+    }
+}