Confirm empty-matrix guard produces "skipped" (not failed) CI on a real push

#663 made push-to-main builds change-aware, which means the build matrix can
now legitimately be empty (e.g. a docs-only merge). GitHub Actions fails
rather than skips a job fed an empty strategy.matrix, so #663 added
if: needs.matrix.outputs.X != '[]' guards to the affected jobs, matching
the pattern already used on the PR side.

The guard's correctness depends on GitHub Actions' skip-cascade behavior: a
job whose needs dependency was skipped (not failed) also skips by default,
via the implicit success() check on a bare if:. This was verified
against GitHub Support statements and community reports, not official
documentation that explicitly confirms it end-to-end for this exact case.

After #663 merges and a product repo picks up the new ref, confirm with a
real docs-only push to a product repo's main:

• The guarded jobs (build-test/merge in bakery-build-native.yml,
  build in bakery-build.yml) report skipped, not failure.

• The readme job in the same run also reports skipped (verifying the
  cascade, not just the direct guard).

• The product repo's CI aggregator (alls-green/ci gate, wherever
  configured) treats that all-skipped run as passing, not failing.

• https://github.com/posit-dev/images-shared/pull/663

• https://github.com/posit-dev/images-shared/issues/652

After pull request #663 merges, use a product repository with the updated ref and make a docs-only push to main. Inspect bakery-build-native.yml, bakery-build.yml, and the configured alls-green/ci gate; done means build-test/merge, build, readme, and the aggregate gate all report skipped or passing rather than failure.