Split publish.mjs into named steps

[sistrall]

Same release, same order, same behaviour. Only the shape changed.

Before: one 160-line main(), with banner comments marking the phases.
After: each phase is a function that owns its step() heading, and main() is the release:

const main = async () => {
  const { distTag } = parseOptions(process.argv.slice(2));
  const branch = preflight(distTag);

  const plan = hasPendingChangesets() ? prepareRelease() : readPublishPlan();

  publishAndTag(distTag);
  push(branch);
  await publishReleaseNotes(plan, distTag);
};

The file now reads in four sections: shell helpers, read-only questions about the repo, the steps, the release.

One structural change. The old bumping flag was tested twice, on either side of the plan read: if (bumping) { build, test, bump } … read plan … if (bumping) commit. Those five are now prepareRelease(), so the fork in main is one ternary naming the two cases — cut a release, or resume one that already bumped and committed.

Smaller ones: the commit subject moved into releaseSubject(plan); the tmpfile handling merged into readPublishPlan, so reading the plan, printing it and refusing an empty one are one function.

Checked: biome clean, and the abort paths still fire (--bogus, --tag with no value, dirty tree). Everything past preflight would need a real release to exercise.