Improve error context for git command helpers

[Phoenix2066]

Summary

This PR improves error messages returned by git command helper methods by
adding contextual wrapping while preserving the original error.

What changed

• Standardized error handling across git command methods
• Wrapped command execution errors with descriptive context using %w
• No changes to command behavior or return values

Why

Previously, many failures surfaced as generic errors (e.g. exit status 128),
which made it difficult to identify which git operation failed.
Adding context improves debuggability without impacting callers.

Scope & compatibility

• No API changes
• No behavior changes
• Existing callers continue to work unchanged

[bakayu]

Hey, thank you for the PR!
I am a little busy right now, will review this on the weekend.