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/app-server-protocol/src/lib.rs

@@ -4,6 +4,7 @@ mod jsonrpc_lite;
 mod protocol;
 mod schema_fixtures;
 
+pub use codex_git_utils::GitSha;
 pub use experimental_api::*;
 pub use export::GenerateTsOptions;
 pub use export::generate_internal_json_schema;