ci: bring the lint gate configs back to current canon

[bryanfawcett]

Config only — no source file and no lint result changes. The canonical
files in nyuchi/.github have moved since this repo adopted the gate, and
these are the files that had drifted:

• .DS_Store
• .yamllint.yaml

What changed upstream, and why each one matters here:

• .yamllint.yaml now ignores lockfiles (pnpm-lock.yaml,
  package-lock.json, yarn.lock and their **/ forms) and Rails fixtures
  (test/fixtures/). Both are generated or templated rather than
  authored: pnpm copies registry deprecation notices into a lockfile
  verbatim at lengths nobody can rewrap, and a Rails fixture is an ERB
  template that is only valid YAML after Rails renders it.
• .github/workflows/lint.yml now carries merge_group:, so required
  checks report on the gh-readonly-queue/** ref in any repo whose
  ruleset uses a merge queue — without it a queued PR stalls for the
  full check_response_timeout and nothing can land. It also widens the
  push trigger to branches: [main, master, scaffold], covering every
  default-branch name in the estate; a repo not listed there fires
  nothing on merge, so the five contexts never reach its
  default-branch head.

Neither applies to this repo today. They are adopted anyway because the
whole point of the canonical files is that they are byte-identical
everywhere — a repo that is "the same except" is how the estate drifted
in the first place.

Re-verified with CI's exact pinned versions (actionlint 1.7.12,
prettier 3.9.4, markdownlint-cli2 0.23.2, yamllint 1.38.0): all five
still clean.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com