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/src/plugins/render_tests.rs

@@ -5,3 +5,19 @@ use pretty_assertions::assert_eq;
 fn render_plugins_section_returns_none_for_empty_plugins() {
     assert_eq!(render_plugins_section(&[]), None);
 }
+
+#[test]
+fn render_plugins_section_includes_descriptions_and_skill_naming_guidance() {
+    let rendered = render_plugins_section(&[PluginCapabilitySummary {
+        config_name: "sample@test".to_string(),
+        display_name: "sample".to_string(),
+        description: Some("inspect sample data".to_string()),
+        has_skills: true,
+        ..PluginCapabilitySummary::default()
+    }])
+    .expect("plugin section should render");
+
+    let expected = "## Plugins\nA plugin is a local bundle of skills, MCP servers, and apps. Below is the list of plugins that are enabled and available in this session.\n### Available plugins\n- `sample`: inspect sample data\n### How to use plugins\n- Discovery: The list above is the plugins available in this session.\n- Skill naming: If a plugin contributes skills, those skill entries are prefixed with `plugin_name:` in the Skills list.\n- Trigger rules: If the user explicitly names a plugin, prefer capabilities associated with that plugin for that turn.\n- Relationship to capabilities: Plugins are not invoked directly. Use their underlying skills, MCP tools, and app tools to help solve the task.\n- Preference: When a relevant plugin is available, prefer using capabilities associated with that plugin over standalone capabilities that provide similar functionality.\n- Missing/blocked: If the user requests a plugin that is not listed above, or the plugin does not have relevant callable capabilities for the task, say so briefly and continue with the best fallback.";
+
+    assert_eq!(rendered, expected);
+}