Align linting with receipt-only package support #53

What to build

Make linting agree with the established receipt-only package behavior introduced in PR #21. A valid project with neither payload nor install scripts must pass lint and remain buildable as a pkgbuild --nopayload package. Other malformed configuration must continue to produce lint errors.

Acceptance criteria

• A valid receipt-only project produces no missing-payload-or-scripts lint error.
• The CLI lint command exits successfully for a valid receipt-only project.
• Receipt-only lint and build behavior is covered together by targeted tests.
• Existing errors for malformed build information and unsafe package names remain intact.
• The unit suite and macOS verification loop pass.

Blocked by

• None — can start immediately.