[connectors] Support connectors part 2 - slash command and tui (#9728)

- [x] Support `/apps` slash command to browse the apps in tui.
- [x] Support inserting apps to prompt using `$`.
- [x] Lots of simplification/renaming from connectors to apps.

codex-rs/core/config.schema.json

@@ -144,6 +144,9 @@
             "apply_patch_freeform": {
               "type": "boolean"
             },
+            "apps": {
+              "type": "boolean"
+            },
             "child_agents_md": {
               "type": "boolean"
             },
@@ -1165,6 +1168,9 @@
         "apply_patch_freeform": {
           "type": "boolean"
         },
+        "apps": {
+          "type": "boolean"
+        },
         "child_agents_md": {
           "type": "boolean"
         },