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/manager_tests.rs

@@ -270,6 +270,73 @@ fn plugin_telemetry_metadata_uses_default_mcp_config_path() {
     );
 }
 
+#[test]
+fn capability_summary_sanitizes_plugin_descriptions_to_one_line() {
+    let codex_home = TempDir::new().unwrap();
+    let plugin_root = codex_home
+        .path()
+        .join("plugins/cache")
+        .join("test/sample/local");
+
+    write_file(
+        &plugin_root.join(".codex-plugin/plugin.json"),
+        r#"{
+  "name": "sample",
+  "description": "Plugin that\n includes   the sample\tserver"
+}"#,
+    );
+    write_file(
+        &plugin_root.join("skills/sample-search/SKILL.md"),
+        "---\nname: sample-search\ndescription: search sample data\n---\n",
+    );
+
+    let outcome = load_plugins_from_config(&plugin_config_toml(true, true), codex_home.path());
+
+    assert_eq!(
+        outcome.plugins[0].manifest_description.as_deref(),
+        Some("Plugin that\n includes   the sample\tserver")
+    );
+    assert_eq!(
+        outcome.capability_summaries()[0].description.as_deref(),
+        Some("Plugin that includes the sample server")
+    );
+}
+
+#[test]
+fn capability_summary_truncates_overlong_plugin_descriptions() {
+    let codex_home = TempDir::new().unwrap();
+    let plugin_root = codex_home
+        .path()
+        .join("plugins/cache")
+        .join("test/sample/local");
+    let too_long = "x".repeat(MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN + 1);
+
+    write_file(
+        &plugin_root.join(".codex-plugin/plugin.json"),
+        &format!(
+            r#"{{
+  "name": "sample",
+  "description": "{too_long}"
+}}"#
+        ),
+    );
+    write_file(
+        &plugin_root.join("skills/sample-search/SKILL.md"),
+        "---\nname: sample-search\ndescription: search sample data\n---\n",
+    );
+
+    let outcome = load_plugins_from_config(&plugin_config_toml(true, true), codex_home.path());
+
+    assert_eq!(
+        outcome.plugins[0].manifest_description.as_deref(),
+        Some(too_long.as_str())
+    );
+    assert_eq!(
+        outcome.capability_summaries()[0].description,
+        Some("x".repeat(MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN))
+    );
+}
+
 #[test]
 fn load_plugins_uses_manifest_configured_component_paths() {
     let codex_home = TempDir::new().unwrap();