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/src/models.rs

@@ -23,7 +23,7 @@ use crate::protocol::SandboxPolicy;
 use crate::protocol::WritableRoot;
 use crate::user_input::UserInput;
 use codex_execpolicy::Policy;
-use codex_git::GhostCommit;
+use codex_git_utils::GhostCommit;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_image::error::ImageProcessingError;
 use schemars::JsonSchema;