#3349 swamp-extensions: CI check that every manifest declares license: and ships a LICENSE file

Follow-up to #3325 (part of the extension license work, #3323–#3328). #3325 adds license: AGPL-3.0-or-later to all 697 first-party manifests and makes codegen write it. Nothing yet stops a new or edited manifest from dropping it, and once #3324 reaches stable a public push without both license: and a LICENSE file fails at publish time instead of before merge.

Change

• Add a verification check (verification/checks.yaml, with its script under scripts/) that fails when any manifest.yaml lacks license:, or lists no non-empty LICENSE, LICENSE.md or LICENSE.txt in additionalFiles.
• Trigger it on any manifest.yaml change and on new extension directories.
• Cover it in scripts/verification_harness_test.ts.

Why separate

It touches trust-root paths (verification/, scripts/), which go through the Review Integrity audit; #3325 stays outside the trust root so it can merge before #3324 reaches stable (Seth, 2026-10-13).

Optional, same change

Repo-level files still describe the old terms: the root LICENSE, README and COPYING-EXCEPTION copy (which still names System Initiative), codegen's own source headers, scripts/ and the root extensions/models/ headers say AGPL v3 with the Swamp Extension and Definition Exception. #3325 changed only shipped files, by decision.