action-check-warnings: emit automated + build-failure, not execution / python-warnings

[mmcky]

QEP-2 requires every bot-opened issue or pull request to carry automated plus at most one diagnostic label. action-check-warnings emits execution and python-warnings, neither of which exists anywhere in the standard, and does not emit automated at all. Carried over from QuantEcon/qeps#3.

The correct contract is automated + build-failure.

This depends on the software widening batch: the repositories the action opens issues in must already carry build-failure, or the emitter creates the label ad hoc and reintroduces exactly the drift QEP-2 exists to end. qe gh bots check (QuantEcon/cli#5) is what verifies the contract afterwards.

Acceptance

• The workflow emits automated + build-failure and no other label
• Open issues it has already opened are re-tagged, so the repositories are conformant rather than merely conformant going forward
• qe gh bots check passes for this emitter