release skill: warn against Update branch on the milestone merge PR #5092

Problem

GitHub always offers Update branch on a milestone's merge PR (v2/main → main), because main always holds commits the merge branch lacks: its own earlier merge commits and any policy syncs made directly to it. The button reads as "this PR is out of date", but clicking it merges main into the merge branch, so the merge PR gains a commit of its own. That breaks the release skill's rule that the merge branch is v2/main (or, for v1.0.0, the release branch) with nothing added: main then receives two merge commits, and the branch head is no longer the commit the release ledger names.

It happened on v1.0.0's merge PR #5090 (#5080): the branch was reset to the release branch's head with git push --force-with-lease.

Expected

Step 3 of .claude/skills/release/SKILL.md warns, next to the "merge commit, never squash" warning, not to click Update branch on the merge PR, explains why it is always offered, and gives the recovery (reset the branch to the release source's head with --force-with-lease, then re-run the git merge-tree check).