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/common.rs

@@ -14,16 +14,6 @@ use serde::Serialize;
 use strum_macros::Display;
 use ts_rs::TS;
 
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema, TS)]
-#[ts(type = "string")]
-pub struct GitSha(pub String);
-
-impl GitSha {
-    pub fn new(sha: &str) -> Self {
-        Self(sha.to_string())
-    }
-}
-
 /// Authentication mode for OpenAI-backed providers.
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, Display, JsonSchema, TS)]
 #[serde(rename_all = "lowercase")]