add @plugin mentions (#13510)

## Note-- added plugin mentions via @, but that conflicts with file
mentions

depends and builds upon #13433.

- introduces explicit `@plugin` mentions. this injects the plugin's mcp
servers, app names, and skill name format into turn context as a dev
message.
- we do not yet have UI for these mentions, so we currently parse raw
text (as opposed to skills and apps which have UI chips, autocomplete,
etc.) this depends on a `plugins/list` app-server endpoint we can feed
the UI with, which is upcoming
- also annotate mcp and app tool descriptions with the plugin(s) they
come from. this gives the model a first class way of understanding what
tools come from which plugins, which will help implicit invocation.

### Tests
Added and updated tests, unit and integration. Also confirmed locally a
raw `@plugin` injects the dev message, and the model knows about its
apps, mcps, and skills.

codex-rs/core/src/plugins/render.rs

@@ -30,6 +30,54 @@ pub(crate) fn render_plugins_section(plugins: &[PluginCapabilitySummary]) -> Opt
     Some(lines.join("\n"))
 }
 
+pub(crate) fn render_explicit_plugin_instructions(
+    plugin: &PluginCapabilitySummary,
+    available_mcp_servers: &[String],
+    available_apps: &[String],
+) -> Option<String> {
+    let mut lines = vec![format!(
+        "Capabilities from the `{}` plugin:",
+        plugin.display_name
+    )];
+
+    if plugin.has_skills {
+        lines.push(format!(
+            "- Skills from this plugin are prefixed with `{}:`.",
+            plugin.display_name
+        ));
+    }
+
+    if !available_mcp_servers.is_empty() {
+        lines.push(format!(
+            "- MCP servers from this plugin available in this session: {}.",
+            available_mcp_servers
+                .iter()
+                .map(|server| format!("`{server}`"))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ));
+    }
+
+    if !available_apps.is_empty() {
+        lines.push(format!(
+            "- Apps from this plugin available in this session: {}.",
+            available_apps
+                .iter()
+                .map(|app| format!("`{app}`"))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ));
+    }
+
+    if lines.len() == 1 {
+        return None;
+    }
+
+    lines.push("Use these plugin-associated capabilities to help solve the task.".to_string());
+
+    Some(lines.join("\n"))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;