feat: add xProof output certification skill

[jasonxkensei]

Add xProof skill -- on-chain output certification

Adds a skill that lets automatons certify their outputs on MultiversX blockchain via the xProof API (https://xproof.app).

What it does

• Hash files locally with sha256sum
• Certify single files via POST /api/proof
• Batch certify up to 50 files via POST /api/batch
• Verify existing proofs via GET /api/proof/:id
• Supports API key auth and x402 (HTTP 402 payment, no account needed)

Why it matters for automatons

An automaton that certifies its outputs creates cryptographic, blockchain-recorded evidence of its work. This builds a verifiable track record -- useful for proving value to creators, clients, and peer agents.

Install

The skill is auto-discovered from skills/xproof/SKILL.md. Alternatively, install at runtime:

# Via the automaton's skill registry
installSkillFromGit("https://github.com/Conway-Research/automaton", "xproof", ...)
Requirements
curl and sha256sum (standard on Linux)
Optional: XPROOF_API_KEY env var (or use x402 payments)
Cost: $0.05 per certification. No new dependencies.