Alpha4: one real preflight run

[daichunghy]

contribkit v0.1.0-alpha.4 is now the current GitHub release. The smallest useful test is a clean clone of the tag, npm ci, npm run verify, then run preflight against a real branch with --base origin/main.

The important failure mode is not a green demo. It is the first confusing or false block in a repository contract. If you try it, report one rule, path, or receipt field that needs to be clearer. This is a request for a real quickstart, not an adoption claim.