meta: migrate to two-branch deploy process

[matildepark]

Treating main as a living project works for constant iteration but it creates scenarios like in #59 where we can't patch stable due to breaking changes in dev.

The changes are small:

• change our workflow to deploy @alpha from a dev branch.
• work on dev predominantly, merging into main when stable, tagging and releasing.

Then we can deploy bug fixes to stable alongside major release work.