feat: add git wrapper package for subprocess operations
Operations:
- Clone, Checkout, Fetch, Pull, Push
- Add, Commit, CommitAll
- Status, CurrentBranch, HasUncommittedChanges, RemoteURL
- Merge, Rebase, AbortMerge, AbortRebase
- CreateBranch, DeleteBranch, Rev, IsAncestor
Error handling:
- Detects ErrNotARepo, ErrMergeConflict, ErrAuthFailure, ErrRebaseConflict
- Wraps git stderr in error messages
Closes gt-u1j.3
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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