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/secrets/Cargo.toml

@@ -11,6 +11,7 @@ workspace = true
 age = { workspace = true }
 anyhow = { workspace = true }
 base64 = { workspace = true }
+codex-git-utils = { workspace = true }
 codex-keyring-store = { workspace = true }
 rand = { workspace = true }
 regex = { workspace = true }