Restore the multi-select quick pick for branch deletion after merge - #9153

The modal from #8947 replaced the branch-deletion quick pick. It caused two problems:

• Delete All appeared under a Remote repository: line, so the button looked like it would delete the repository.
• Deleting both branches without Delete All took two trips through the dialog.

This PR reverts the modal and brings back the quick pick. The revert is partial: unrelated fixes from the same commit stay in.

Quick pick restored (PullRequestReviewCommon.deleteBranch)

• showQuickPick again, with canPickMany: true and ignoreFocusOut: true.
• The githubPullRequests.defaultDeletionMethod.* settings again set which options start ticked. The remote branch is always ticked.
• The repository moves to the remote branch item's description, so the list only contains things that can be deleted:

  ☑ Delete remote branch origin/feature   https://github.com/owner/repo
  ☑ Delete local branch feature
  

• Two small differences from the pre-0.168.0 version:
  • The label falls back to the bare branch name when the head remote isn't found, so it no longer shows undefined/<branch>.
  • The description uses the head repository's name, not the base repository's. This only matters for forks with a different name.

Settings

• package.nls.json: the defaultDeletionMethod.* descriptions go back to describing what's ticked by default.

Kept from later or unrelated work

• Skip redundant remote-branch deletion prompts after merge #8989: remoteBranchDeletedOnMerge. When GitHub has already deleted the remote branch on merge, the post-merge prompt doesn't offer it again.
• Replace branch deletion picker with a modal dialog #8947, fixes that don't depend on the modal:
  • The webviews now get a reply to pr.deleteBranch, so the Delete Branch... button no longer stays disabled after a successful delete.
  • A remoteHead deletion now marks the remote branch as deleted in the PR page.
  • A local branch that no longer exists is treated as deleted instead of failing.

Tests

• folderRepositoryManager.test.ts and pullRequestOverview.test.ts: the modal tests are rewritten against showQuickPick, covering the same cases.
• New test: which options start ticked follows the defaultDeletionMethod.* settings.

For reviewers

This isn't a literal git revert. If a full revert is preferred, dropping the three #8947 fixes is a small change.

• Fixes Branch deletion modal (0.168.0): "Delete All" reads as deleting the repository, and single deletes need two trips #9060