[Stage 0.3] Establish independent v1 and v2 CI, preview, and release workflows

[draedful]

Parent: #310

Goal

Give v1 and v2 independent, explicit delivery paths before the v2 branch is opened for migration work.

Scope

• Run the agreed CI suite for pull requests and relevant pushes on both main and the future v2 branch.
• Keep main and v2 Storybook or documentation previews on distinct destinations.
• Replace the shared conditional release workflow with two explicit workflows:
  • release-v1.yml
  • release-v2.yml
• Configure the v1 workflow before cutover:
  • stable branch: main
  • permitted versions: 1.x
  • npm dist-tag: latest
• Prepare the same v1 workflow for the future maintenance line:
  • branch: release/v1
  • permitted versions: 1.x
  • npm dist-tag: v1
• Configure the v2 workflow before cutover:
  • branch: v2
  • permitted versions: 2.x prereleases
  • npm dist-tag: next
• Design the v2 workflow so that cutover changes only its branch and stable-channel configuration:
  • branch: main
  • permitted versions: stable 2.x
  • npm dist-tag: latest
• Add branch, version, and dist-tag guards that fail before publication.
• Provide dry-run or equivalent release-routing verification.
• Keep publication behind an explicit release pull request or approval step.
• Confirm that the release GitHub App can operate under the planned rulesets with only the minimum required bypass.

Acceptance criteria

• CI is configured for both main and v2.
• Main and v2 previews cannot overwrite one another.
• Separate v1 and v2 release workflows exist.
• Before cutover, a non-1.x release from main fails before publishing.
• Before cutover, the v2 workflow targets v2, permits only 2.x prereleases, and cannot update latest.
• The future release/v1 route is present before that branch is created.
• The post-cutover main/latest configuration for the v2 workflow is documented and testable.
• Release routing can be verified without an accidental publication.
• Publication always requires a reviewed release pull request or equivalent approval.