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/protocol/v1.rs

@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 use std::path::PathBuf;
 
+use codex_git_utils::GitSha;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::ReasoningSummary;
@@ -21,7 +22,6 @@ use serde::Serialize;
 use ts_rs::TS;
 
 use crate::protocol::common::AuthMode;
-use crate::protocol::common::GitSha;
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]