[Cutover 3] Promote v2 to main and switch the stable release channels

[draedful]

Goal

Make the approved v2 release line the repository default and stable release source while keeping v1 independently maintainable.

Scope

• Ensure old or conflicting release pull requests are completed or closed.
• Include the post-cutover workflow configuration in the reviewed v2 promotion pull request:
  • release-v2.yml: main, stable 2.x, latest
  • release-v1.yml: release/v1, 1.x, v1
• Promote v2 to main through a reviewed pull request.
• Do not force-push or replace main history.
• Wait for post-merge CI before approving a stable v2 release.
• Create and separately approve the stable v2 release pull request.
• Publish stable 2.x under latest.
• Verify versions, tags, changelog, GitHub Release, npm artifacts, and dist-tags.
• Install the stable v2 packages from npm in isolated consumer fixtures.
• Update rulesets, previews, documentation, issue templates, and contribution routing.
• Keep v2 during the observation window, then remove it through the protected-branch process.
• Document rollback:
  • before npm publication: reviewed revert of the promotion merge or fix-forward;
  • after npm publication: restore latest to the last approved package, then use a reviewed revert or fix-forward because a published version cannot be removed safely.

Acceptance criteria

• main contains the approved v2 promotion and remains the default branch.
• Post-merge CI passes before stable publication.
• Stable 2.x is published only after separate release approval.
• release-v2.yml owns main/2.x/latest.
• release-v1.yml owns release/v1/1.x/v1.
• Stable v2 artifacts pass isolated npm consumer checks.
• Both rollback scenarios are documented.
• The observation window completes before v2 is removed.