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/plugins.rs

@@ -26,6 +26,7 @@ use wiremock::MockServer;
 
 const SAMPLE_PLUGIN_CONFIG_NAME: &str = "sample@test";
 const SAMPLE_PLUGIN_DISPLAY_NAME: &str = "sample";
+const SAMPLE_PLUGIN_DESCRIPTION: &str = "inspect sample data";
 
 fn sample_plugin_root(home: &TempDir) -> std::path::PathBuf {
     home.path().join("plugins/cache/test/sample/local")
@@ -36,7 +37,9 @@ fn write_sample_plugin_manifest_and_config(home: &TempDir) -> std::path::PathBuf
     std::fs::create_dir_all(plugin_root.join(".codex-plugin")).expect("create plugin manifest dir");
     std::fs::write(
         plugin_root.join(".codex-plugin/plugin.json"),
-        format!(r#"{{"name":"{SAMPLE_PLUGIN_DISPLAY_NAME}"}}"#),
+        format!(
+            r#"{{"name":"{SAMPLE_PLUGIN_DISPLAY_NAME}","description":"{SAMPLE_PLUGIN_DESCRIPTION}"}}"#
+        ),
     )
     .expect("write plugin manifest");
     std::fs::write(
@@ -225,6 +228,14 @@ async fn plugin_skills_append_to_instructions() -> Result<()> {
         instructions_text.contains("`sample`"),
         "expected enabled plugin name in instructions"
     );
+    assert!(
+        instructions_text.contains("`sample`: inspect sample data"),
+        "expected plugin description in instructions"
+    );
+    assert!(
+        instructions_text.contains("skill entries are prefixed with `plugin_name:`"),
+        "expected plugin skill naming guidance"
+    );
     assert!(
         instructions_text.contains("sample:sample-search: inspect sample data"),
         "expected namespaced plugin skill summary"