Simple directory mentions (#14970)

- Adds simple support for directory mentions in the TUI.
- Codex App/VS Code will require minor change to recognize a directory
mention as such and change the link behavior.
- Directory mentions have a trailing slash to differentiate from
extensionless files


<img width="972" height="382" alt="image"
src="https://github.com/user-attachments/assets/8035b1eb-0978-465b-8d7a-4db2e5feca39"
/>
<img width="978" height="228" alt="image"
src="https://github.com/user-attachments/assets/af22cf0b-dd10-4440-9bee-a09915f6ba52"
/>

codex-rs/tui_app_server/src/bottom_pane/chat_composer.rs

@@ -7268,6 +7268,7 @@ mod tests {
             vec![FileMatch {
                 score: 1,
                 path: PathBuf::from("src/main.rs"),
+                match_type: codex_file_search::MatchType::File,
                 root: PathBuf::from("/tmp"),
                 indices: None,
             }],