chore(#846): remove unused PR-size labeler (size/S–XL) workflow (#848)

The PR Gate workflow's only job, size-check, labeled every PR with
size/S–size/XL based on lines changed. Those labels aren't used in any
review, triage, or automation flow, so the workflow was pure noise.

- Delete .github/workflows/pr-gate.yml
- Drop size-check from required status checks in both rulesets so PRs
  don't block forever on a check that never reports
- Remove pr-gate.yml from INERT_WORKFLOWS (ci-test-scope.cjs) and the
  knownInert list (ci-test-scope.test.cjs)
- Remove "PR Gate / size-check" from setup-branch-protection.sh

Closes #846

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit 606363c416)

tests/ci-test-scope.test.cjs

@@ -343,7 +343,7 @@ describe('INERT_WORKFLOWS allowlist integrity guard', () => {
   const knownInert = [
     'stale.yml', 'branch-cleanup.yml', 'branch-naming.yml', 'auto-label-issues.yml',
     'auto-branch.yml', 'auto-backmerge.yml', 'close-draft-prs.yml',
-    'dismiss-unauthorized-pr-approvals.yml', 'pr-gate.yml', 'pr-target-validator.yml',
+    'dismiss-unauthorized-pr-approvals.yml', 'pr-target-validator.yml',
     'pr-template-format.yml', 'require-issue-link.yml', 'changeset-required.yml',
     'docs-required.yml', 'discord-changelog.yml',
   ];