[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/protocol/src/models.rs

@@ -674,7 +674,7 @@ impl From<Vec<UserInput>> for ResponseInputItem {
                         image_index += 1;
                         local_image_content_items_with_label_number(&path, Some(image_index))
                     }
-                    UserInput::Skill { .. } => Vec::new(), // Skill bodies are injected later in core
+                    UserInput::Skill { .. } | UserInput::Mention { .. } => Vec::new(), // Tool bodies are injected later in core
                 })
                 .collect::<Vec<ContentItem>>(),
         }

codex-rs/protocol/src/user_input.rs

@@ -29,6 +29,8 @@ pub enum UserInput {
         name: String,
         path: std::path::PathBuf,
     },
+    /// Explicit mention selected by the user (name + app://connector id).
+    Mention { name: String, path: String },
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS, JsonSchema)]