[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/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)]