feat: Add plugin share checkout (#22435)

Adds plugin/share/checkout to turn a shared remote plugin into a local
working copy under ~/plugins/<name>.

Registers the copy in the managed personal marketplace and records the
remote-to-local mapping for later share/save flows.

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -7,7 +7,6 @@ use codex_plugin::PluginId;
 use codex_plugin::PluginIdError;
 use codex_protocol::protocol::Product;
 use codex_utils_absolute_path::AbsolutePathBuf;
-use dirs::home_dir;
 use serde::Deserialize;
 use serde_json::Value as JsonValue;
 use std::fs;
@@ -225,6 +224,16 @@ pub fn list_marketplaces(
     list_marketplaces_with_home(additional_roots, home_dir().as_deref())
 }
 
+pub(crate) fn home_dir() -> Option<PathBuf> {
+    ["HOME", "USERPROFILE"]
+        .into_iter()
+        .filter_map(std::env::var_os)
+        .filter(|value| !value.is_empty())
+        .map(PathBuf::from)
+        .find(|path| path.is_absolute())
+        .or_else(dirs::home_dir)
+}
+
 pub fn validate_marketplace_root(root: &Path) -> Result<String, MarketplaceError> {
     let Some(path) = find_marketplace_manifest_path(root) else {
         return Err(MarketplaceError::InvalidMarketplaceFile {