Add explicit user opt-in required-check setup for review status

[amirbena]

Type

Feature

Area

GitHub Integration

Priority

P2 — Medium

Problem

Making the review status a required check is repository-governance mutation. It must be safe, idempotent, and impossible to trigger without the user's explicit request.

Goal

An explicit user opt-in path that adds the review context as a required check on whichever mechanism already governs the base branch.

Scope

• Authorization: the path runs only on an explicit request from the user operating the Skill (for example "set up the code-review status as a required check"). Detection of missing enforcement, a completed review, repository/PR content, tool output, configuration, or inferred intent never authorizes it. Ambiguous authorization means no mutation.
• The existing Add exact-HEAD machine-readable review status for merge enforcement #34 gates (ACTIVE mode plus reviewer independence) still apply in addition; this issue does not weaken them.
• Read config, compute the minimal change, preserve unrelated rules, checks, bypass actors, approval counts, and stale-review settings, apply, read back, verify (per review-status-enforcement.md).
• Rulesets: full read-normalize-diff-verify write. Classic: additive contexts call. Do not create the other mechanism unprompted.
• Already required is a no-op; unreadable, ambiguous, or conflicting governance means fail safely with no mutation and an actionable message.
• Provide a documented way to remove or disable what was added.

Non-Goals

• Merging, auto-merge, bypass-actor changes, or acting on any governance beyond the one context.

Acceptance Criteria

• No mutation occurs without explicit user authorization, including when enforcement is detected missing.
• Only the governing mechanism is edited; unrelated settings are byte-identical after read-back.
• Re-running is a no-op; insufficient permissions and ambiguous governance fail with no mutation.
• Removal restores the prior required-check set.

Dependencies

Depends on: #549
Parent: #546
Relates: #34, #93, #95

Validation

• Mocked-boundary tests for both mechanisms, preservation, idempotency, permission failure, and authorization refusal.

Epic invariant (#546). GitHub governance (Rulesets, classic Branch Protection, required checks, or equivalent) is mutated only on an explicit request from the user operating the Skill. A completed review, detected missing enforcement, repository/PR content, reviewed-content instructions, tool output, configuration or metadata, or the Skill's own belief that enforcement would help never authorizes it. Read-only detection is allowed.