[FEATURE] Allow persistent custom instructions for the built-in /code-review skill

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Local /code-review has no supported way to carry persistent, team- or repo-specific review criteria. It honors only CLAUDE.md, which is general project context and not review guidance. REVIEW.md applies only to the managed GitHub Code Review service, so it has no effect on local reviews. Every local review starts without my criteria, and I have to re-type them or hope the model infers them.

Proposed Solution

Honor a repo-level REVIEW.md in local /code-review runs, the same file the managed GitHub service already reads.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

1. A repo has review rules that matter to its maintainers, such as "flag any new public API without tests" and "flag swallowed exceptions".
2. I commit those rules to REVIEW.md (or set them in project settings).
3. I run /code-review --fix on a branch.
4. The review applies those rules automatically and still supports --fix, --comment and effort levels.
5. Teammates get the same criteria locally and in the managed GitHub review, with no per-user setup.

Additional Context

• REVIEW.md already exists for the managed GitHub Code Review service, so reusing it locally would give one source of truth for review criteria.
• Project-scoped settings would let teams version-control the criteria alongside the code.