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/Cargo.toml

@@ -22,6 +22,7 @@ codex-cloud-tasks-client = { path = "../cloud-tasks-client", features = [
 ] }
 codex-client = { workspace = true }
 codex-core = { path = "../core" }
+codex-git-utils = { workspace = true }
 codex-login = { path = "../login" }
 codex-tui = { path = "../tui" }
 codex-utils-cli = { workspace = true }