Retire dev: work and release on main #24

Work lands on dev and reaches main through a dev → main pull request, enforced by the PR base gate. A topic branch should target main directly, and release-plz should open its release PR against main.

Done when: main is the default branch, the base gate is gone, the one required check on main is an aggregate CI result, release-plz runs on pushes to main only, and dev is deleted with its ruleset.