ci: rename branch-protection caller job id to branch-protection

[rubenvdlinde]

GitHub names a reusable-workflow status context <caller-job-id> / <called-job-name>.

This repo's caller job id was not branch-protection, so the shared branch-protection
workflow reported under the wrong context name. The org rulesets (Main Branch Protection
id 14128365, Beta Branch Protection id 14128357) require exactly:

branch-protection / check-branch
quality / Quality Report

No name match means the required context never reported at all — not as a failure, as
nothing. The PR sits at BLOCKED forever, and the absence of the check is visually identical to
the check merely not having finished yet. That is the whole defect: a check's absence looks
exactly like its success is still pending.

This renames the caller job id so the context matches. It does not change what is required and
does not touch any ruleset or branch protection — that is a governance decision, not a code
one.

Irony worth recording: this PR itself has to be admin-merged, because the defect it fixes is
precisely what makes the required context unreportable on the current tip.

[rubenvdlinde]

Superseded — the change is already on main.

.github/workflows/branch-protection.yml on main already declares the caller job as branch-protection:, which is what this PR proposed. Verified by reading the file on origin/main; the check on my own PRs reports as branch-protection / check-branch, the context name this rename existed to produce.

Closing so the open-PR list shows work that still needs doing.