(ci): declare js-yaml as a devDependency, state the asset count of a release without macOS #931

Follow-up of #930 (#899) review.

• test/build-workflow.test.js requires js-yaml, which is not in package.json: it is hoisted from electron-updater, app-builder-lib and @eslint/eslintrc. A dependency update that stops hoisting it breaks the test with MODULE_NOT_FOUND. Add js-yaml to devDependencies (lock regenerated session-side with npx npm@10 --package-lock-only).
• docs/releasing.md step 5 counts 19 assets for a full build only. A release drafted by hand without macOS has fewer (derived, not measured: 10 — minus the 4 dmg/zip, 4 blockmaps and latest-mac.yml). Measure on the next mac-less draft and state the count.

no-changelog.