ci: add Prepare release PR workflow

Automates the "Prepare for X.Y.Z release" commit and release PR against
master. The workflow sets `nextBump` in version-policies.json (so Rush
bumps to the right target on publish), then asks Claude to draft the PR
body from the commits on the release branch. Per-package version bumps
and CHANGELOGs are still produced by `rush version --bump` and `rush
publish --apply` during publish.yml — not here.

Idempotent on re-run (HEAD-subject check); supports a dry_run input that
prints the planned diff and PR body without pushing. See
.github/RELEASE.md for the runbook.