Add marketplace command (#17087)

Added a new top-level `codex marketplace add` command for installing
plugin marketplaces into Codex’s local marketplace cache.

This change adds source parsing for local directories, GitHub shorthand,
and git URLs, supports optional `--ref` and git-only `--sparse` checkout
paths, stages the source in a temp directory, validates the marketplace
manifest, and installs it under
`$CODEX_HOME/marketplaces/<marketplace-name>`

Included tests cover local install behavior in the CLI and marketplace
discovery from installed roots in core. Scoped formatting and fix passes
were run, and targeted CLI/core tests passed.

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

@@ -2,6 +2,7 @@ use super::LoadedPlugin;
 use super::PluginLoadOutcome;
 use super::PluginManifestPaths;
 use super::curated_plugins_repo_path;
+use super::installed_marketplaces::installed_marketplace_roots_from_config;
 use super::load_plugin_manifest;
 use super::manifest::PluginManifestInterface;
 use super::marketplace::MarketplaceError;
@@ -874,7 +875,8 @@ impl PluginsManager {
         }
 
         let (installed_plugins, enabled_plugins) = self.configured_plugin_states(config);
-        let marketplace_outcome = list_marketplaces(&self.marketplace_roots(additional_roots))?;
+        let marketplace_outcome =
+            list_marketplaces(&self.marketplace_roots(config, additional_roots))?;
         let mut seen_plugin_keys = HashSet::new();
         let marketplaces = marketplace_outcome
             .marketplaces
@@ -1218,10 +1220,18 @@ impl PluginsManager {
         (installed_plugins, enabled_plugins)
     }
 
-    fn marketplace_roots(&self, additional_roots: &[AbsolutePathBuf]) -> Vec<AbsolutePathBuf> {
+    fn marketplace_roots(
+        &self,
+        config: &Config,
+        additional_roots: &[AbsolutePathBuf],
+    ) -> Vec<AbsolutePathBuf> {
         // Treat the curated catalog as an extra marketplace root so plugin listing can surface it
         // without requiring every caller to know where it is stored.
         let mut roots = additional_roots.to_vec();
+        roots.extend(installed_marketplace_roots_from_config(
+            config,
+            self.codex_home.as_path(),
+        ));
         let curated_repo_root = curated_plugins_repo_path(self.codex_home.as_path());
         if curated_repo_root.is_dir()
             && let Ok(curated_repo_root) = AbsolutePathBuf::try_from(curated_repo_root)