Problem

Three-way merge exists but there are no cherry-pick or rebase operations. Server-side PR merge strategies (squash-merge, rebase-merge) cannot be implemented.

Impact

• PR merge limited to merge commits only
• Cannot implement squash-and-merge or rebase-and-merge
• Cannot programmatically cherry-pick hotfixes across branches

Acceptance Criteria

• cherry_pick(repo, commit_sha, onto_ref) — apply a single commit’s diff onto a branch
• rebase(repo, branch, onto) — replay a sequence of commits onto a new base
• Conflict detection and reporting (same format as merge conflicts)
• Supports squash mode (combine multiple commits into one)
• Tests covering: clean cherry-pick, conflict detection, multi-commit rebase