feat: warn when a scan reads zero files

[ded-furby]

Summary

• warn on stderr when a scan resolves zero files so clean output is not mistaken for a clean codebase
• add CLI coverage for the zero-files warning path and the normal scan path
• closes Warn when a scan resolves zero files #20

Scope

• This change is focused on one issue or one small improvement.
• I updated docs or examples if behavior changed.
• I added or updated tests for code changes.

Validation

npm test
npm run typecheck:tests
npm run build

Notes for maintainers

• The warning is advisory only; it does not change exit-code behavior for --fail-on.
• The warning includes extra guidance for --changed scans because an empty diff is another likely cause.