Add public proof playground magnets

[zrk222]

What changed

• Added a no-signup, deterministic Proof Playground to the public Space.
• Added four copyable agent recipes for readiness, MVP creation, Graph Ops, and independent verification.
• Added Cursor, OpenCode, and MCP setup links with the local read-only authority boundary.
• Added clipboard fallback and reduced-motion handling.

Why

The public surface now gives visitors an immediate proof moment and a low-friction path to the local product instead of only a static tour.

Verification

• python -m pytest -q: 493 passed, 2 skipped
• prestige audit deploy/huggingface/index.html: 100/100
• prestige critique: P1 0, P2 0, P3 0
• prestige render-audit: PASS
• prestige challenge: 3/3 mutations killed
• Hugging Face publish workflow: success

The preview is explicitly browser-only and makes no model call, source upload, credential access, deployment, or production claim.