Move git utilities into a dedicated crate (#15564)

- create `codex-git-utils` and move the shared git helpers into it with
file moves preserved for diff readability
- move the `GitInfo` helpers out of `core` so stacked rollout work can
depend on the shared crate without carrying its own git info module

---------

Co-authored-by: Ahmed Ibrahim <219906144+aibrahim-oai@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

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

@@ -2,9 +2,9 @@ use super::PluginManifestInterface;
 use super::load_plugin_manifest;
 use super::store::PluginId;
 use super::store::PluginIdError;
-use crate::git_info::get_git_repo_root;
 use codex_app_server_protocol::PluginAuthPolicy;
 use codex_app_server_protocol::PluginInstallPolicy;
+use codex_git_utils::get_git_repo_root;
 use codex_protocol::protocol::Product;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use dirs::home_dir;