setup-node provides Node but not Yarn; the workflow was relying on the runner
image shipping it, as GitHub's hosted images do. When the act_runner image
changed, `yarn install` began failing with exit 127 and every push since
6964ce7 (2026-07-19) silently stopped deploying — a dead pipeline reads the
same as a site that did not need to change.
Pinned to 1.x: the lockfile is v1 and `--frozen-lockfile` is v1 syntax.
Serve it as text/plain on deploy so browsers display the raw text
instead of downloading; add .dockerignore so local image builds can't
ship stale _site or host node_modules.
