feat(github-app): issue-side slop triage

[JSONbored]

Parent roadmap: #525
Parent phase: #526

Background

A large share of Gittensor slop arrives as issues, not PRs. The CI today only surfaces on PRs. Issue advisories are already built but never published.

Goal

Publish a managed, quiet-by-default issue triage panel for maintainers.

Current Behavior

buildIssueAdvisory runs on issue webhooks (src/queue/processors.ts) but no public surface is published for issues.

Desired Behavior

• Add maybePublishIssuePublicSurface gated by a new issueTriageMode setting (off / comment / label_only, default off).
• Reuse buildIssueQualityReport + buildCollisionReport and the existing managed-comment writer in src/github/comments.ts.
• Quiet by default: only comment when issue quality is hold / do_not_use or a duplicate cluster exists.

Implementation Requirements

• Mirror the PR public-surface skip logic; settings + migration for issueTriageMode.

Public/Private Output Boundaries

• Issue panel shows duplicate refs + quality status with reasons; never contributor reward/trust internals.

Acceptance Criteria

• High-traffic trackers stay quiet; only low-quality / duplicate incoming issues get a panel.

Testing Requirements

• npm run test:ci, 97%+ coverage. Tests for the quiet-by-default thresholds + public-safe content.