feat(cli): discover .continue/checks/ in cn review (#11697)

* feat(cli): discover .continue/checks/ in cn review

The README and quickstart docs tell users to create check files in
.continue/checks/, but cn review only discovered files from
.continue/agents/. This caused a broken onboarding path where users
following the docs would get zero reviews when testing locally.

resolveFromLocal() now reads from both .continue/agents/ and
.continue/checks/, deduplicating by filename. Files in agents/ take
precedence if the same filename exists in both directories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(cli): correct readdirSync mock types in resolveReviews tests

Cast mock implementations with `as typeof fs.readdirSync` instead of
returning `as unknown as fs.Dirent[]`. The CI lint step runs
`tsc --noEmit` which caught the type mismatch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>