Stricter CI check to catch common mistakes early on

* Confirm the `metadata` field exists in the Blueprint place
* Confirm it's not a direct copy of another Blueprint from the repo – people often copy&paste those and merging such a PR would lead to a  Gallery entry that looks like a duplicate
* Confirm the `author` field is the same as the PR opener's GitHub username

cc @bgrgicak would you be able to help with that?