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/protocol/Cargo.toml

@@ -13,7 +13,7 @@ workspace = true
 
 [dependencies]
 codex-execpolicy = { workspace = true }
-codex-git = { workspace = true }
+codex-git-utils = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
 codex-utils-image = { workspace = true }
 icu_decimal = { workspace = true }