Import enabled plugins from known marketplaces (#31672)

## Summary

- discover enabled-plugin marketplace sources from the user-level known
marketplace registry
- preserve scoped settings and supported Git/directory declarations,
falling back to the matching registry `installLocation` for file, URL,
npm-hosted, and inline marketplace sources
- resolve relative registry materialization paths against the
external-agent home
- reuse the existing marketplace add and plugin install flow during
import
- emit `plugin_not_found` tracing and analytics with plugin and
marketplace identity

## Why

Enabled plugins can remain in settings after their marketplace source
has moved into the user-level registry. Import detection previously
skipped those plugins because it only consulted settings-defined
marketplace sources.

## Impact

`/import` can now restore those marketplaces and install their enabled
plugins through the normal plugin installer, including npm, Git, and
local plugin sources. Missing plugins remain visible in import results
and the plugin-install failure analytics stream.

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

@@ -57,6 +57,7 @@ use crate::store::PluginStore;
 use crate::store::PluginStoreError;
 use crate::tool_suggest_metadata::ToolSuggestMetadataCache;
 use codex_analytics::AnalyticsEventsClient;
+use codex_analytics::PluginInstallSource;
 use codex_config::ConfigLayerStack;
 use codex_config::clear_user_plugin;
 use codex_config::set_user_plugin_enabled;
@@ -370,6 +371,7 @@ pub struct PluginsManager {
     restriction_product: Option<Product>,
     auth_mode: RwLock<Option<AuthMode>>,
     analytics_events_client: RwLock<Option<AnalyticsEventsClient>>,
+    plugin_install_source: PluginInstallSource,
 }
 
 #[derive(Clone)]
@@ -449,9 +451,15 @@ impl PluginsManager {
             restriction_product,
             auth_mode: RwLock::new(auth_mode),
             analytics_events_client: RwLock::new(None),
+            plugin_install_source: PluginInstallSource::Manual,
         }
     }
 
+    pub fn with_plugin_install_source(mut self, source: PluginInstallSource) -> Self {
+        self.plugin_install_source = source;
+        self
+    }
+
     pub fn set_auth_mode(&self, auth_mode: Option<AuthMode>) -> bool {
         let mut stored_auth_mode = match self.auth_mode.write() {
             Ok(auth_mode_guard) => auth_mode_guard,
@@ -1419,6 +1427,7 @@ impl PluginsManager {
         if let Some(analytics_events_client) = analytics_events_client {
             analytics_events_client.track_plugin_install_failed(
                 self.telemetry_metadata_for_plugin_id(plugin_id),
+                self.plugin_install_source,
                 error_type.to_string(),
             );
         }