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/codex.rs

@@ -235,7 +235,6 @@ pub(crate) struct PreviousTurnSettings {
 
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::feedback_tags;
-use crate::git_info::get_git_repo_root;
 use crate::guardian::GuardianReviewSessionManager;
 use crate::hook_runtime::PendingInputHookDisposition;
 use crate::hook_runtime::inspect_pending_input;
@@ -350,6 +349,7 @@ use crate::unified_exec::UnifiedExecProcessManager;
 use crate::util::backoff;
 use crate::windows_sandbox::WindowsSandboxLevelExt;
 use codex_async_utils::OrCancelExt;
+use codex_git_utils::get_git_repo_root;
 use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
 use codex_otel::metrics::names::THREAD_STARTED_METRIC;