Codex task: implement capability discovery artifact buttons now

[GsCommand]

Implement Issue #201 directly.

Patch public/capabilities.html to add these buttons to the Build Plan panel:

• Copy agent card
• Copy ENS records
• Copy MCP surface

Add JS helpers:

• agentCard() returns JSON for selected build plan
• ensRecords() returns TXT record suggestions
• discoverySurface() returns JSON draft

Wire buttons to clipboard and show the existing toast.

Keep static only. No backend. No checkout. No fake live claims. Preserve all existing build-plan behavior.

Acceptance:

• Existing export/copy SDK buttons still work.
• New three buttons copy correct output.
• Empty build plan returns useful placeholder values.
• No console errors.