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/slash_command.rs

@@ -44,6 +44,7 @@ pub enum SlashCommand {
     Side,
     Btw,
     Copy,
+    Export,
     Raw,
     Diff,
     Mention,
@@ -96,6 +97,7 @@ impl SlashCommand {
             SlashCommand::App => "continue this session in the Desktop app",
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
             SlashCommand::Copy => "copy last response as markdown",
+            SlashCommand::Export => "export the conversation as markdown",
             SlashCommand::Raw => "toggle raw scrollback mode for copy-friendly terminal selection",
             SlashCommand::Diff => "show git diff (including untracked files)",
             SlashCommand::Mention => "mention a file",
@@ -163,6 +165,7 @@ impl SlashCommand {
                 | SlashCommand::Ide
                 | SlashCommand::Keymap
                 | SlashCommand::Mcp
+                | SlashCommand::Export
                 | SlashCommand::Raw
                 | SlashCommand::Usage
                 | SlashCommand::Pets
@@ -178,6 +181,7 @@ impl SlashCommand {
         matches!(
             self,
             SlashCommand::Copy
+                | SlashCommand::Export
                 | SlashCommand::Raw
                 | SlashCommand::Diff
                 | SlashCommand::Mention
@@ -196,6 +200,7 @@ impl SlashCommand {
             | SlashCommand::Fork
             | SlashCommand::Init
             | SlashCommand::Compact
+            | SlashCommand::Export
             | SlashCommand::Keymap
             | SlashCommand::Vim
             | SlashCommand::ElevateSandbox