Executable API house rules with vacuum #13

What to build

Turn the estate's API standard from a document into an executable gate, using vacuum alongside the oasdiff breaking-change check this action already runs.

The rules land one per pull request, each at error severity, each shipping together with the fixes for its own violations. main is never red, and every rule is provably able to fail the moment it merges — rather than sitting at warn where nobody reads it.

Rule A (every operation has an operationId) is already satisfied by all three specs, so it ships first as the skeleton.

Acceptance criteria

• The action exposes lint inputs symmetric with its existing breaking-change inputs
• Each rule merges green, with its violations fixed in the same change
• Every rule is demonstrated to fail against a deliberately bad fixture before it is trusted to pass
• No rule is left at warn

Blocked by

None — can start immediately.