feat: add maintainer-only `skip-review` label guard to Agentic PR review workflow

[katriendg]

Summary

The automated PR review workflow (.github/workflows/pr-review.md) currently has no way for maintainers to bypass automated review for a specific PR. When maintainers need to fast-track a merge — such as for urgent fixes, release preparation, or already-reviewed content — they must wait for the automated review cycle to complete or manually dismiss the review.

Problem Statement

There is no mechanism for maintainers to signal that automated review should be skipped for a PR. A skip-review label cannot be respected by the Activation Guard today because the guard only checks draft status.

Proposed Solution

Add a dual-condition guard to the Activation Guard section of pr-review.md:

• The PR has the skip-review label, AND
• The PR author's association is MEMBER, OWNER, or COLLABORATOR

When both conditions are true, the agent calls noop immediately with a message indicating the skip was triggered by a maintainer. This prevents automated review from blocking or flagging the PR.

If the skip-review label is present but the PR author is not a maintainer, the label is ignored and the full review proceeds. This ensures the guard cannot be triggered by non-maintainers even if they somehow add the label.

Acceptance Criteria

• The skip-review label is created in the repository with a distinct color and description
• The Activation Guard in pr-review.md checks for the skip-review label as a new exit condition
• The skip is only triggered when BOTH the label is present AND the PR author is MEMBER, OWNER, or COLLABORATOR
• A non-maintainer PR with the skip-review label proceeds through the full review, unchanged
• The noop call message is descriptive: "Skipping: skip-review label set by maintainer."
• The lock file (pr-review.lock.yml) is regenerated via gh aw compile

Out of Scope

• Changes to who can apply labels in GitHub (a separate repository settings concern)
• Changes to the dependency PR review workflow

Implementation Notes

• The skip-review label does not yet exist in the repo — it must be created (the existing needs-revision and review-passed labels are present)
• skip-review is read by the Activation Guard as an input label; it is never written by the workflow, so the add-labels safe-output allowlist does not need to change
• Only pr-review.md requires editing; the lock file is auto-generated
• The Activation Guard already follows a checklist pattern — the new condition is a single bullet addition
• The existing Maintainer Advisory Mode already establishes the MEMBER/OWNER/COLLABORATOR vocabulary; this guard reuses the same author association logic
• No new safe-outputs permissions are required — noop with max: 1 is already declared