Set up CD

Every time a PR is merged to the `mergit` repo, a successful CI run on `main` should redeploy the merge bot.

The deployment should be zero down-time, with the new instance spun up and ready to accept connections before switching over and destroying the old instance.

The issue does not name any files, tests, deployment provider, or existing CI configuration. Start by locating the CI workflow for main and the current merge-bot deployment entry point; done means a successful main CI run redeploys the bot and the new instance accepts connections before the old one is destroyed.

Written by the indexing model from the issue text.