Add a Gitea Actions test-status badge plus a short README "CI" section
covering how to read the check and what to do when it's red. Capture
the (out-of-tree) branch-protection rule on `main` in docs/ci.md so
the gate that requires the test check is reproducible from the repo
alone — covers both the Gitea UI path and the equivalent API call.
Refs: PRD 0002
Assisted-by: Claude Code
