canon: support single-branch repositories #1

What

Let canon run in a repository with one long-lived branch, usually main.

Why

Setup refuses a config where the integration and production branches are the same, so a repository with only main can't use the plugin. Many small projects work that way.

Approach

A first idea: allow branches.integration to be null. With no integration branch:

• /canon:staging opens the pull request into main and stops at the button, the way /canon:production does. protect-merge already refuses a merge into production, so nothing merges without the owner.
• /canon:production has nothing to release and says so.
• protect-branch and protect-push guard main alone.
• /canon:production loses its check for commits that skipped a pull request, which depends on merge commits into the integration branch.

Open question: is one branch a separate mode in /canon:setup, or just the same questions with "none" as an answer?