Add explicit issue relevance review

[robert-crandall]

Problem

An open issue is not necessarily work that still needs doing. Ranking it does not answer whether it has already been addressed, superseded, or become unnecessary. I want to investigate that question in this app instead of opening a separate Copilot session for each issue.

Expected behavior

• Offer an explicit Check relevance action, separate from priority assessment and publishing. Investigate the issue, discussion, linked PRs and current code; disclose missing evidence and incomplete coverage.
• Save a recommendation: still needed, already addressed, superseded / no longer needed, or unclear. Include supporting evidence, when it was checked, and questions that need a human answer. Age alone is not evidence of irrelevance; implemented code does not necessarily satisfy the original need.
• Let me accept or correct the recommendation and explicitly rerun the review when needed. Keep the model's recommendation distinguishable from my decision.
• Do not automatically close issues, mark tasks Done, publish work, or hide uncertain candidates as a consequence of the review.

This should work for the personal queue without any project board. It also supplies the grooming step before service-level prioritization and publication in #65.