Move marketplace add under plugin command (#18116)

## Summary
- move the marketplace add CLI from `codex marketplace add` to `codex
plugin marketplace add`
- keep marketplace config overrides working through the nested plugin
command
- reject `--sparse` for local marketplace directory sources before the
local-source install path bypasses git-source validation

## Validation
- `just fmt`
- `git diff --check`
- `cargo test -p codex-cli`
- `cargo test -p codex-core marketplace_add -- --nocapture`
- `cargo test -p codex-core
install_plugin_updates_config_with_relative_path_and_plugin_key --
--nocapture`
- `xli-test-marketplace-cli` local isolated matrix: `T1`, `L1`-`L10`

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

@@ -77,6 +77,11 @@ where
         sparse_paths,
     } = request;
     let source = parse_marketplace_source(&source, ref_name)?;
+    if !sparse_paths.is_empty() && !matches!(source, MarketplaceSource::Git { .. }) {
+        return Err(MarketplaceAddError::InvalidRequest(
+            "--sparse is only supported for git marketplace sources".to_string(),
+        ));
+    }
 
     let install_root = marketplace_install_root(codex_home);
     fs::create_dir_all(&install_root).map_err(|err| {
@@ -287,6 +292,38 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn add_marketplace_sync_rejects_sparse_checkout_for_local_directory_source() -> Result<()> {
+        let codex_home = TempDir::new()?;
+        let source_root = TempDir::new()?;
+        write_marketplace_source(source_root.path(), "local copy")?;
+
+        let err = add_marketplace_sync_with_cloner(
+            codex_home.path(),
+            MarketplaceAddRequest {
+                source: source_root.path().display().to_string(),
+                ref_name: None,
+                sparse_paths: vec![".agents".to_string()],
+            },
+            |_url, _ref_name, _sparse_paths, _destination| {
+                panic!("git cloner should not be called for local marketplace sources")
+            },
+        )
+        .unwrap_err();
+
+        assert_eq!(
+            err.to_string(),
+            "--sparse is only supported for git marketplace sources"
+        );
+        assert!(
+            !codex_home
+                .path()
+                .join(codex_config::CONFIG_TOML_FILE)
+                .exists()
+        );
+        Ok(())
+    }
+
     #[test]
     fn add_marketplace_sync_treats_existing_local_directory_source_as_already_added() -> Result<()>
     {