fix: Deduplicate installed local and remote curated plugins (#25681)

## Summary
- Deduplicate installed `openai-curated` and `openai-curated-remote`
plugin conflicts by feature flag.
- Prefer remote when remote plugins are enabled; otherwise prefer local,
while preserving one-sided installs.

## Testing
- `just fmt`
- `git diff --check`
- Targeted `just test` was blocked locally because `cargo-nextest` is
not installed.

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

@@ -5,6 +5,7 @@ use crate::manifest::load_plugin_manifest;
 use crate::marketplace::MarketplacePluginSource;
 use crate::marketplace::list_marketplaces;
 use crate::marketplace::load_marketplace;
+use crate::remote::REMOTE_GLOBAL_MARKETPLACE_NAME;
 use crate::remote::RemoteInstalledPlugin;
 use crate::store::PluginStore;
 use crate::store::plugin_version_for_source;
@@ -113,10 +114,15 @@ pub async fn load_plugins_from_layer_stack(
     extra_plugins: HashMap<String, PluginConfig>,
     store: &PluginStore,
     restriction_product: Option<Product>,
+    prefer_remote_curated_conflicts: bool,
 ) -> PluginLoadOutcome<McpServerConfig> {
     let skill_config_rules = skill_config_rules_from_stack(config_layer_stack);
-    let mut configured_plugins = configured_plugins_from_stack(config_layer_stack);
-    configured_plugins.extend(extra_plugins);
+    let configured_plugins = merge_configured_plugins_with_remote_installed(
+        configured_plugins_from_stack(config_layer_stack),
+        extra_plugins,
+        store,
+        prefer_remote_curated_conflicts,
+    );
     let mut configured_plugins: Vec<_> = configured_plugins.into_iter().collect();
     configured_plugins.sort_unstable_by(|(a, _), (b, _)| a.cmp(b));
 
@@ -149,6 +155,61 @@ pub async fn load_plugins_from_layer_stack(
     PluginLoadOutcome::from_plugins(plugins)
 }
 
+fn merge_configured_plugins_with_remote_installed(
+    mut configured_plugins: HashMap<String, PluginConfig>,
+    extra_plugins: HashMap<String, PluginConfig>,
+    store: &PluginStore,
+    prefer_remote_curated_conflicts: bool,
+) -> HashMap<String, PluginConfig> {
+    let local_curated_installed_plugin_keys = configured_plugins
+        .keys()
+        .filter_map(|plugin_key| {
+            installed_plugin_name_for_marketplace(
+                plugin_key,
+                OPENAI_CURATED_MARKETPLACE_NAME,
+                store,
+            )
+            .map(|plugin_name| (plugin_name, plugin_key.clone()))
+        })
+        .collect::<HashMap<_, _>>();
+
+    for (plugin_key, plugin_config) in extra_plugins {
+        let remote_curated_plugin_name = installed_plugin_name_for_marketplace(
+            &plugin_key,
+            REMOTE_GLOBAL_MARKETPLACE_NAME,
+            store,
+        );
+        let local_curated_plugin_key = remote_curated_plugin_name
+            .as_ref()
+            .and_then(|plugin_name| local_curated_installed_plugin_keys.get(plugin_name));
+
+        if let Some(local_curated_plugin_key) = local_curated_plugin_key {
+            if prefer_remote_curated_conflicts {
+                configured_plugins.remove(local_curated_plugin_key);
+            } else {
+                continue;
+            }
+        }
+
+        configured_plugins.insert(plugin_key, plugin_config);
+    }
+
+    configured_plugins
+}
+
+fn installed_plugin_name_for_marketplace(
+    plugin_key: &str,
+    marketplace_name: &str,
+    store: &PluginStore,
+) -> Option<String> {
+    let plugin_id = PluginId::parse(plugin_key).ok()?;
+    if plugin_id.marketplace_name != marketplace_name {
+        return None;
+    }
+    store.active_plugin_root(&plugin_id)?;
+    Some(plugin_id.plugin_name)
+}
+
 pub fn remote_installed_plugins_to_config(
     plugins: &[RemoteInstalledPlugin],
     store: &PluginStore,