chore: clarify plugin + app copy in model instructions (#14541)

- clarify app mentions are in user messages
- clarify what it means for tools to be provided via `codex_apps` MCP
- add plugin descriptions (with basic sanitization) to top-level `##
Plugins` section alongside the corresponding plugin names
- explain that skills from plugins are prefixed with `plugin_name:` in
top-level `##Plugins` section

changes to more logically organize `Apps`, `Skills`, and `Plugins`
instructions will be in a separate PR, as that shuffles dev + user
instructions in ways that change tests broadly.

### Tests
confirmed in local rollout, some new tests.

codex-rs/core/tests/suite/client.rs

@@ -971,7 +971,7 @@ async fn includes_apps_guidance_as_developer_message_for_chatgpt_auth() {
     let request = resp_mock.single_request();
     let request_body = request.body_json();
     let input = request_body["input"].as_array().expect("input array");
-    let apps_snippet = "Apps are mentioned in the prompt in the format";
+    let apps_snippet = "Apps are mentioned in user messages in the format";
 
     let has_developer_apps_guidance = input.iter().any(|item| {
         item.get("role").and_then(|value| value.as_str()) == Some("developer")