GeezerVison

…h actions between them

- Dropping branch S onto branch T opens a menu with: compare S with T, merge S
  into T (when T is current), rebase / interactive rebase S on T (when S is
  current) and create a PR from S into T (local branches resolve to their upstream)
- Works within and across the local and remote branch trees
- While dragging, the source row is faded, the row under the pointer is
  highlighted and a label (`source → target`) follows the pointer
- Add `Remote.TryGetCreatePullRequestURL` overload with an explicit target
  branch (cross-fork PRs supported for GitHub)

Co-Authored-By: Claude Opus 5.5 <[email protected]>

…nches

- Branch labels in the commit graph can be dragged onto other branch labels in
  the graph or onto branches in the branch trees (and vice versa), opening the
  same context menu with the actions between the two branches
- While dragging, the dragged label in the graph is faded and the label under
  the pointer is highlighted
- Move the drag state, pointer label and branch-pair context menu out of
  `BranchTree` into a shared `BranchDragDrop` helper
- Add `CommitRefsPresenter.HitTestDecorator` for precise label hit-testing

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>