Align skill artifact distribution with the Cloudflare RFC

[Aaronontheweb]

Parent epic: #84

Add archive artifact support for skills with resources while preserving existing per-file resource routes.

Tasks:

• Generate deterministic archives for resourceful skill versions.
• Add archive download endpoint.
• Emit type archive in the RFC feed for resourceful skills.
• Keep type skill-md for single-file skills.
• Preserve existing resource URLs for compatibility.

Acceptance criteria:

• RFC feed artifact digests verify the exact bytes at url.
• Archive entries contain SKILL.md at the archive root.
• Path traversal and absolute paths are rejected.
• Integration tests cover skill-md and archive entries.

[Aaronontheweb]

Completed by #95.