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/app-server/tests/suite/v2/thread_metadata_update.rs

@@ -20,6 +20,7 @@ use codex_app_server_protocol::ThreadStartResponse;
 use codex_app_server_protocol::ThreadStatus;
 use codex_core::ARCHIVED_SESSIONS_SUBDIR;
 use codex_core::state_db::reconcile_rollout;
+use codex_git_utils::GitSha;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::GitInfo as RolloutGitInfo;
 use codex_state::StateRuntime;
@@ -378,7 +379,7 @@ async fn thread_metadata_update_can_clear_stored_git_fields() -> Result<()> {
         "Thread preview",
         Some("mock_provider"),
         Some(RolloutGitInfo {
-            commit_hash: Some("abc123".to_string()),
+            commit_hash: Some(GitSha::new("abc123")),
             branch: Some("feature/sidebar-pr".to_string()),
             repository_url: Some("git@example.com:openai/codex.git".to_string()),
         }),