[codex] Describe uninstalled cross-repo plugin reads (#18449)

## Summary
- Populate `PluginDetail.description` in core for uninstalled cross-repo
plugins when detailed fields are unavailable until install.
- Include the source Git URL plus optional path/ref/sha details in that
fallback description.
- Keep `details_unavailable_reason` as the structured signal while
app-server forwards the description normally.
- Add plugin-read coverage proving the response does not clone the
remote source just to show the message.

## Why
Uninstalled cross-repo plugins intentionally return sparse detail data
so listing/reading does not clone the plugin source. Without a
description, Desktop and TUI detail pages look like an ordinary empty
plugin. This gives users a concrete explanation and source pointer while
keeping the existing structured reason available for callers.

## Validation
- `just fmt`
- `cargo test -p codex-core
read_plugin_for_config_uninstalled_git_source_requires_install_without_cloning`
- `cargo test -p codex-app-server plugin_read --test all`
- `just fix -p codex-core`
- `just fix -p codex-app-server`

Note: `cargo test -p codex-app-server` was also attempted before the
latest refactor and failed broadly in unrelated v2
thread/realtime/review/skills suites; the new plugin-read test passed in
that run as well.

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

@@ -1058,10 +1058,11 @@ impl PluginsManager {
             })?;
         let plugin_key = plugin_id.as_key();
         if matches!(plugin.source, MarketplacePluginSource::Git { .. }) && !plugin.installed {
+            let description = remote_plugin_install_required_description(&plugin.source);
             return Ok(PluginDetail {
                 id: plugin_key,
                 name: plugin.name,
-                description: None,
+                description: Some(description),
                 source: plugin.source,
                 policy: plugin.policy,
                 interface: plugin.interface,
@@ -1495,6 +1496,34 @@ impl PluginsManager {
     }
 }
 
+fn remote_plugin_install_required_description(source: &MarketplacePluginSource) -> String {
+    let source_description = match source {
+        MarketplacePluginSource::Git {
+            url,
+            path,
+            ref_name,
+            sha,
+        } => {
+            let mut parts = vec![url.clone()];
+            if let Some(path) = path {
+                parts.push(format!("path `{path}`"));
+            }
+            if let Some(ref_name) = ref_name {
+                parts.push(format!("ref `{ref_name}`"));
+            }
+            if let Some(sha) = sha {
+                parts.push(format!("sha `{sha}`"));
+            }
+            parts.join(", ")
+        }
+        MarketplacePluginSource::Local { path } => path.as_path().display().to_string(),
+    };
+
+    format!(
+        "This is a cross-repo plugin. Install it to view more detailed information. The source of the plugin is {source_description}."
+    )
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum PluginInstallError {
     #[error("{0}")]

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

@@ -1623,7 +1623,13 @@ plugins = true
         Some(PluginDetailsUnavailableReason::InstallRequiredForRemoteSource)
     );
     assert!(!outcome.plugin.installed);
-    assert!(outcome.plugin.description.is_none());
+    let expected_description = format!(
+        "This is a cross-repo plugin. Install it to view more detailed information. The source of the plugin is {missing_remote_repo_url}, path `plugins/toolkit`."
+    );
+    assert_eq!(
+        outcome.plugin.description.as_deref(),
+        Some(expected_description.as_str())
+    );
     assert!(outcome.plugin.skills.is_empty());
     assert!(outcome.plugin.apps.is_empty());
     assert!(outcome.plugin.mcp_server_names.is_empty());