fix: honor skipChecks in Vitest helpers

[dacharyc]

Summary

Fixes #133.

Both Vitest helpers resolved skipChecks from configuration but omitted it when building the options for runChecks. The runner already supported explicit exclusions; the helpers now forward config.skipChecks as skipCheckIds.

• Add 10 regression tests covering both helpers with inline, directory-loaded, and default-loaded configuration, plus existing check selection, curated pages, and sampling behavior.
• Include the helper module in coverage instead of excluding it.
• Change explicit-skip result wording to Check explicitly skipped, independent of the entry point, and update runner/dependency assertions.
• Prefer skipChecks in CI guidance so newly added checks remain enabled; remove the obsolete helper limitation.

Verification

• Confirmed the regression tests fail before the fix because skipCheckIds is absent, and pass afterward.
• Full suite: 1,776 tests pass across 68 files.
• Focused helper/config/runner/dependency suite: 116 tests pass.
• npm run version:check, npm run lint, npm run format:check, and npm run build pass.

No check implementations or scoring rules changed.