If git merge --abort throws, the repo is left mid-merge. Previously the
code logged a warning and returned a conflict result, giving the UI a
stale file list. Now it returns Blocked with an explicit message so the
caller knows manual resolution is required.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Add GitService.CheckoutBranchAsync; compare targetBranch to current HEAD
before MergeNoFfAsync and switch when they differ. Returns Blocked if the
branch does not exist. Add three new tests (two service, one GitService).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
