fix(review): record structured acceptance evidence without PR body notes #5577

User Story

As an OpenShell maintainer reviewing pull requests, I need acceptance decisions to be recorded consistently without adding administrative explanations to the author's PR description.

Problem Statement

PR #5575 includes an unstructured note under Related Issue explaining that its linked issue lacks state:accepted and its author has ADMIN access. The note was present when the PR opened. The contributor PR skill directs authors with sufficient access to note the missing label, while Review Readiness independently applies state:accepted to qualifying PRs. GitHub label events support an intent.rationale field, already used by the issue-labeling tools, so record the acceptance basis there in a structured format rather than inserting prose in the PR description.

Impact / Why This Matters

Administrative notes clutter the Related Issue section and can become stale. A consistent, discoverable acceptance reason helps explain automated labeling without implying that acceptance is a review approval.

Acceptance Criteria

• Contributor guidance does not instruct authors to add acceptance or permission explanations to the PR body; the actual issue link and acceptance gate remain intact.
• Automatically applied state:accepted has a consistently structured, discoverable reason that identifies the qualifying basis without modifying the PR description.
• Preserve existing acceptance eligibility and reviewer-routing behavior; no new unstructured PR annotation is added.
• Cover the qualifying cases and unsuccessful lookups with appropriate tests and update relevant contributor guidance.

Reproduction Steps

1. Inspect the Related Issue section of fix(review): show the cause of Review Readiness failures in annotations #5575 and its state:accepted label.
2. Read the contributor PR skill's access verification instructions and the Review Readiness accepted-work labeling flow.

Environment

• OpenShell contributor workflow and GitHub Actions Review Readiness