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/src/lib.rs

@@ -4,6 +4,7 @@ use std::path::PathBuf;
 use std::sync::Arc;
 
 use anyhow::Result;
+use codex_git_utils::get_git_repo_root;
 use codex_keyring_store::DefaultKeyringStore;
 use codex_keyring_store::KeyringStore;
 use schemars::JsonSchema;
@@ -161,22 +162,6 @@ pub fn environment_id_from_cwd(cwd: &Path) -> String {
     format!("cwd-{short}")
 }
 
-fn get_git_repo_root(base_dir: &Path) -> Option<PathBuf> {
-    let mut dir = base_dir.to_path_buf();
-
-    loop {
-        if dir.join(".git").exists() {
-            return Some(dir);
-        }
-
-        if !dir.pop() {
-            break;
-        }
-    }
-
-    None
-}
-
 pub(crate) fn compute_keyring_account(codex_home: &Path) -> String {
     let canonical = codex_home
         .canonicalize()