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-protocol/schema/json/ServerNotification.json

@@ -964,6 +964,13 @@
       ],
       "type": "object"
     },
+    "FuzzyFileSearchMatchType": {
+      "enum": [
+        "file",
+        "directory"
+      ],
+      "type": "string"
+    },
     "FuzzyFileSearchResult": {
       "description": "Superset of [`codex_file_search::FileMatch`]",
       "properties": {
@@ -981,6 +988,9 @@
             "null"
           ]
         },
+        "match_type": {
+          "$ref": "#/definitions/FuzzyFileSearchMatchType"
+        },
         "path": {
           "type": "string"
         },
@@ -995,6 +1005,7 @@
       },
       "required": [
         "file_name",
+        "match_type",
         "path",
         "root",
         "score"