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/git-utils/src/errors.rs

@@ -0,0 +1,35 @@
+use std::path::PathBuf;
+use std::process::ExitStatus;
+use std::string::FromUtf8Error;
+
+use thiserror::Error;
+use walkdir::Error as WalkdirError;
+
+/// Errors returned while managing git worktree snapshots.
+#[derive(Debug, Error)]
+pub enum GitToolingError {
+    #[error("git command `{command}` failed with status {status}: {stderr}")]
+    GitCommand {
+        command: String,
+        status: ExitStatus,
+        stderr: String,
+    },
+    #[error("git command `{command}` produced non-UTF-8 output")]
+    GitOutputUtf8 {
+        command: String,
+        #[source]
+        source: FromUtf8Error,
+    },
+    #[error("{path:?} is not a git repository")]
+    NotAGitRepository { path: PathBuf },
+    #[error("path {path:?} must be relative to the repository root")]
+    NonRelativePath { path: PathBuf },
+    #[error("path {path:?} escapes the repository root")]
+    PathEscapesRepository { path: PathBuf },
+    #[error("failed to process path inside worktree")]
+    PathPrefix(#[from] std::path::StripPrefixError),
+    #[error(transparent)]
+    Walkdir(#[from] WalkdirError),
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+}