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/core/src/realtime_context.rs

@@ -1,10 +1,10 @@
 use crate::codex::Session;
 use crate::compact::content_items_to_text;
 use crate::event_mapping::is_contextual_user_message_content;
-use crate::git_info::resolve_root_git_project_for_trust;
 use crate::truncate::TruncationPolicy;
 use crate::truncate::truncate_text;
 use chrono::Utc;
+use codex_git_utils::resolve_root_git_project_for_trust;
 use codex_protocol::models::ResponseItem;
 use codex_state::SortKey;
 use codex_state::ThreadMetadata;