The ext-sdk-bundle-rollout flag reached 100%, so stable releases now ship a
standalone build of main through ext-vscode-publish-stable.yml rather than the
combined legacy+next A/B VSIX. Rewrite the skill around that path and record
what is left to retire.
Also corrects two things the old text got wrong: nightly is manual-dispatch
only (the cron was removed because PublishNightly gained required reviewers,
so unattended runs sat waiting and starved their successors), and the stable
workflow tags before it builds, so a tag-push failure fails early rather than
leaving a published-but-untagged release.
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