Research ruleset and branch-protection integration

## Type

Research

## Area

GitHub Integration

## Priority

P2 — Medium

## Problem

`skills/github-pr-review/policies/review-status-enforcement.md` (shipped via
#34 / PR #112) already documents an ENFORCED / NOT ENFORCED / UNKNOWN
detection approach ("Enforcement-state detection (read-only)") and
mutation-safety constraints for making the status a required check
("Explicit opt-in required-check setup"). What it does not contain is a
comparative analysis of repository rulesets vs classic branch protection
for this specific use case, and it cites no official GitHub documentation.

## Goal

Write the rulesets-vs-classic-branch-protection comparison for configuring
a required status check, and cite official GitHub docs.

## Scope

- Compare repository rulesets and classic branch protection specifically
for adding one required-status-check context: capabilities, API surface,
precedence when both exist on the same branch, and any migration/
compatibility considerations.
- Cite official GitHub documentation (docs.github.com) for both
mechanisms and for the required-status-checks configuration surface.

## Non-Goals

- Re-deriving the enforcement-state detection approach or mutation-safety
constraints already documented in `review-status-enforcement.md`
("Enforcement-state detection (read-only)" and "Explicit opt-in
required-check setup") — those are done; do not re-ask for them.
- Performing any mutation.

## Acceptance Criteria

- [ ] Rulesets vs classic branch protection are compared for
required-status-check configuration specifically.
- [ ] Artifact cites official GitHub docs (docs.github.com).

## Dependencies

Depends on: none
Relates: #34
Blocks: #95
Parent: #49

## Validation

- Research artifact cites official GitHub docs.