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)

scripts/ci-test-scope.cjs

@@ -20,7 +20,6 @@ const INERT_WORKFLOWS = new Set([
   'auto-backmerge.yml',
   'close-draft-prs.yml',
   'dismiss-unauthorized-pr-approvals.yml',
-  'pr-gate.yml',
   'pr-target-validator.yml',
   'pr-template-format.yml',
   'require-issue-link.yml',

scripts/setup-branch-protection.sh

@@ -61,13 +61,11 @@ REQUIRED_CHECKS_MAIN=(
   "security-scan"
   "Changeset Required / changeset-lint"
   "Docs Required / docs-lint"
-  "PR Gate / size-check"
   "Validate Branch Name / check-branch"
 )
 
 REQUIRED_CHECKS_NEXT=(
   "test"
-  "PR Gate / size-check"
   "Validate Branch Name / check-branch"
   "Changeset Required / changeset-lint"
   "Docs Required / docs-lint"