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/state/src/extract.rs

@@ -61,7 +61,7 @@ fn apply_session_meta_from_item(metadata: &mut ThreadMetadata, meta_line: &Sessi
         metadata.cwd = meta_line.meta.cwd.clone();
     }
     if let Some(git) = meta_line.git.as_ref() {
-        metadata.git_sha = git.commit_hash.clone();
+        metadata.git_sha = git.commit_hash.as_ref().map(|sha| sha.0.clone());
         metadata.git_branch = git.branch.clone();
         metadata.git_origin_url = git.repository_url.clone();
     }

codex-rs/state/src/runtime/threads.rs

@@ -1089,7 +1089,7 @@ mod tests {
                 memory_mode: None,
             },
             git: Some(GitInfo {
-                commit_hash: Some("rollout-sha".to_string()),
+                commit_hash: Some(codex_git_utils::GitSha::new("rollout-sha")),
                 branch: Some("rollout-branch".to_string()),
                 repository_url: Some("git@example.com:openai/codex.git".to_string()),
             }),