GitHub comments: show linked galleries with direct attachments

[zachdunn]

In plain terms

Update the existing managed GitHub attachments comment so it can present linked galleries alongside legacy direct PR/issue attachments.

Experience

The comment should clearly distinguish galleries from loose gh/... attachments and link back to each stable gallery page. A PR or issue may have several linked galleries.

Acceptance criteria

• Existing marker-owned comments continue to update in place
• Existing direct-attachment workflows remain compatible
• Multiple linked galleries render clearly
• Gallery titles and URLs are escaped safely
• GitHub comment failure never rolls back a completed upload or gallery mutation
• put --comment and uploads comment behavior is documented
• Tests cover galleries only, loose attachments only, both, and empty results
• The CLI skill, package documentation, and changeset are updated

Depends on

GitHub linking and the public gallery page.