Merge staging into main - #27

pyproject depends on unify as a sibling path, matching the local layout, so
poetry install has failed in CI since the job was written — every run has
reported failure before reading a single example. The adjacent black job was
given a workaround for the same cause in July; this one was left. Check both
repos out side by side and run from the docs one.

The suite executes every python and shell block in every .mdx against the live
API, so it is metered work: it now runs on the release pull request and on
demand rather than on every push, where the examples are usually untouched and
the bill would not be. black keeps its push trigger, and stays the required
check it already was.

Poetry is installed directly: knowsuchagency/poetry-install takes no
working-directory, which the two-checkout layout needs, and still runs on
node12.