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.rs

@@ -66,6 +66,7 @@ const DEFAULT_APP_CONFIG_FILE: &str = ".app.json";
 const OPENAI_CURATED_MARKETPLACE_NAME: &str = "openai-curated";
 const REMOTE_PLUGIN_SYNC_TIMEOUT: Duration = Duration::from_secs(30);
 static CURATED_REPO_SYNC_STARTED: AtomicBool = AtomicBool::new(false);
+const MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN: usize = 1024;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct AppConnectorId(pub String);
@@ -191,7 +192,7 @@ impl PluginCapabilitySummary {
                 .manifest_name
                 .clone()
                 .unwrap_or_else(|| plugin.config_name.clone()),
-            description: plugin.manifest_description.clone(),
+            description: prompt_safe_plugin_description(plugin.manifest_description.as_deref()),
             has_skills: !plugin.skill_roots.is_empty(),
             mcp_server_names,
             app_connector_ids: plugin.apps.clone(),
@@ -213,6 +214,23 @@ impl PluginCapabilitySummary {
     }
 }
 
+fn prompt_safe_plugin_description(description: Option<&str>) -> Option<String> {
+    let description = description?
+        .split_whitespace()
+        .collect::<Vec<_>>()
+        .join(" ");
+    if description.is_empty() {
+        return None;
+    }
+
+    Some(
+        description
+            .chars()
+            .take(MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN)
+            .collect(),
+    )
+}
+
 #[derive(Debug, Clone, PartialEq)]
 pub struct PluginLoadOutcome {
     plugins: Vec<LoadedPlugin>,