ci : push release tag explicitly in release.yml (#27261)
Add a "Create and push git tag" step to the release job, right before
the "Create release" step. The tag is created with git tag and pushed
with the deploy key already configured by the Clone step, instead of
relying on the Releases API (action-create-release) to create it as a
side effect.
The tag is lightweight, matching all existing b<number> release tags.
The step is idempotent: if the tag already exists (e.g. on a re-run),
creation and push are skipped.
Assisted-by: pi:llama.cpp/Qwen3.8-27B
