chore: merge git crates (#5909)

Merge `git-apply` and `git-tooling` into `utils/`

codex-rs/utils/git/src/platform.rs

@@ -0,0 +1,37 @@
+use std::path::Path;
+
+use crate::GitToolingError;
+
+#[cfg(unix)]
+pub fn create_symlink(
+    _source: &Path,
+    link_target: &Path,
+    destination: &Path,
+) -> Result<(), GitToolingError> {
+    use std::os::unix::fs::symlink;
+
+    symlink(link_target, destination)?;
+    Ok(())
+}
+
+#[cfg(windows)]
+pub fn create_symlink(
+    source: &Path,
+    link_target: &Path,
+    destination: &Path,
+) -> Result<(), GitToolingError> {
+    use std::os::windows::fs::FileTypeExt;
+    use std::os::windows::fs::symlink_dir;
+    use std::os::windows::fs::symlink_file;
+
+    let metadata = std::fs::symlink_metadata(source)?;
+    if metadata.file_type().is_symlink_dir() {
+        symlink_dir(link_target, destination)?;
+    } else {
+        symlink_file(link_target, destination)?;
+    }
+    Ok(())
+}
+
+#[cfg(not(any(unix, windows)))]
+compile_error!("codex-git symlink support is only implemented for Unix and Windows");