Documentation: Add `cn review` command documentation

[continue]

Summary

PR #11697 added the ability for cn review to discover check files from both .continue/agents/ and .continue/checks/ directories. However, the cn review command is not documented in the CLI documentation at all.

Current state

• The CLI documentation (docs/cli/) does not mention the cn review command
• The checks documentation references /check skill for local running but not cn review
• Users who follow the docs create checks in .continue/checks/ but may not know about cn review

Documentation needed

1. Add documentation for the cn review command to the CLI docs (possibly in a new docs/cli/review.mdx file or in the quickstart)
2. Document that cn review discovers checks from both:
   • .continue/agents/*.md
   • .continue/checks/*.md (with .continue/agents/ taking precedence on duplicate filenames)
3. Update the checks documentation (docs/checks/running-locally.mdx) to mention cn review as an alternative to the /check skill

Related

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

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Co-Authored-By: Continue noreply@continue.dev