fix(plugins) rm plugin descriptions (#23254)

## Summary
Removes Plugin descriptions from the dev message, since descriptions of
skills and MCPs cover the capabilities offered by the plugin.

## Testing
- [x] Updates unit tests

codex-rs/core/src/context/available_plugins_instructions.rs

@@ -40,27 +40,17 @@ impl ContextualUserFragment for AvailablePluginsInstructions {
     fn body(&self) -> String {
         let mut lines = vec![
             "## Plugins".to_string(),
-            "A 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.".to_string(),
-            "### Available plugins".to_string(),
+            "A plugin is a local bundle of skills, MCP servers, and apps.".to_string(),
         ];
 
-        lines.extend(
-            self.plugins
-                .iter()
-                .map(|plugin| match plugin.description.as_deref() {
-                    Some(description) => format!("- `{}`: {description}", plugin.display_name),
-                    None => format!("- `{}`", plugin.display_name),
-                }),
-        );
-
         lines.push("### How to use plugins".to_string());
         lines.push(
-            r###"- Discovery: The list above is the plugins available in this session.
-- Skill naming: If a plugin contributes skills, those skill entries are prefixed with `plugin_name:` in the Skills list.
+            r###"- Skill naming: If a plugin contributes skills, those skill entries are prefixed with `plugin_name:` in the Skills list.
+- MCP naming: Plugin-provided MCP tools keep standard MCP identifiers such as `mcp__server__tool`; use tool provenance to tell which plugin they come from.
 - Trigger rules: If the user explicitly names a plugin, prefer capabilities associated with that plugin for that turn.
 - Relationship to capabilities: Plugins are not invoked directly. Use their underlying skills, MCP tools, and app tools to help solve the task.
-- Preference: When a relevant plugin is available, prefer using capabilities associated with that plugin over standalone capabilities that provide similar functionality.
-- 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."###
+- Relevance: Determine what a plugin can help with from explicit user mention or from the plugin-associated skills, MCP tools, and apps exposed elsewhere in this turn.
+- Missing/blocked: If the user requests a plugin that does not have relevant callable capabilities for the task, say so briefly and continue with the best fallback."###
                 .to_string(),
         );