docs(vcs): merging a stacked pull request without closing it #843

Problem

While merging a stack, the base branch of #819 was deleted through the API after #818 merged. GitHub closed #819 instead of retargeting it, because it only retargets when the branch is deleted by the merge itself. The branch had to be recreated at its old SHA, the PR reopened and retargeted, and then rebased, because #818 had been squashed.

Scope

aidd_docs/memory/vcs.md: how a stacked PR is merged. Retarget the dependent PR to next first (gh pr edit --base next), then merge its base. Never delete a base branch that another open PR targets. After a squash merge of the base, rebase the dependent PR with git rebase --onto next <old-base-head>.

Acceptance criteria

• vcs.md carries the rule in one place, and no other page restates it.