Move PR checkout flow to dialog and drop /checkout-pr command

- Replace the inline composer PR menu with a dedicated `PullRequestThreadDialog`
- Remove `/checkout-pr` from composer slash-command parsing and tests
- Fix branch selector virtualization sizing for the checkout-PR row
d28ced8fd7Julius Marminge committed on 3/10/2026, 5:50:45 AM· committed by Julius Marmingeparent0766080
6 files changedLine totals unavailable