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/app-server/tests/suite/fuzzy_file_search.rs

@@ -257,6 +257,7 @@ async fn test_fuzzy_file_search_sorts_and_includes_indices() -> Result<()> {
                 {
                     "root": root_path.clone(),
                     "path": "abexy",
+                    "match_type": "file",
                     "file_name": "abexy",
                     "score": 84,
                     "indices": [0, 1, 2],
@@ -264,6 +265,7 @@ async fn test_fuzzy_file_search_sorts_and_includes_indices() -> Result<()> {
                 {
                     "root": root_path.clone(),
                     "path": sub_abce_rel,
+                    "match_type": "file",
                     "file_name": "abce",
                     "score": expected_score,
                     "indices": [4, 5, 7],
@@ -271,6 +273,7 @@ async fn test_fuzzy_file_search_sorts_and_includes_indices() -> Result<()> {
                 {
                     "root": root_path.clone(),
                     "path": "abcde",
+                    "match_type": "file",
                     "file_name": "abcde",
                     "score": 71,
                     "indices": [0, 1, 4],