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/cloud-tasks/src/lib.rs

@@ -10,6 +10,8 @@ pub use cli::Cli;
 use anyhow::anyhow;
 use chrono::Utc;
 use codex_cloud_tasks_client::TaskStatus;
+use codex_git_utils::current_branch_name;
+use codex_git_utils::default_branch_name;
 use owo_colors::OwoColorize;
 use owo_colors::Stream;
 use std::cmp::Ordering;
@@ -119,11 +121,11 @@ struct RealGitInfo;
 #[async_trait::async_trait]
 impl GitInfoProvider for RealGitInfo {
     async fn default_branch_name(&self, path: &std::path::Path) -> Option<String> {
-        codex_core::git_info::default_branch_name(path).await
+        default_branch_name(path).await
     }
 
     async fn current_branch_name(&self, path: &std::path::Path) -> Option<String> {
-        codex_core::git_info::current_branch_name(path).await
+        current_branch_name(path).await
     }
 }