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/tui/src/onboarding/trust_directory.rs

@@ -1,7 +1,7 @@
 use std::path::PathBuf;
 
 use codex_core::config::set_project_trust_level;
-use codex_core::git_info::resolve_root_git_project_for_trust;
+use codex_git_utils::resolve_root_git_project_for_trust;
 use codex_protocol::config_types::TrustLevel;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;