feat: add xProof certification skills

[jasonxkensei]

Add xProof certification skills

Summary

Adds xproof_skills.ts -- off-chain certification for agent outputs via the xProof API, with a composite flow that chains xProof certification with MX-8004 Validation Registry submit_proof.

Ref: multiversx/mx-openclaw-skills#1, issue #679

What it does

Function	Description
certifyFile(params)	Hash a local file (SHA-256) and certify on MultiversX via xProof
certifyHash(params)	Certify a pre-computed hash (no local file needed)
certifyBatch(params)	Certify up to 50 files in a single API call
verifyProof(certId)	Check certification status and blockchain details
certifyAndSubmitProof(params)	Composite: xProof certify + Validation Registry submit_proof in one call

Integration architecture

Agent Output
    |
    v
certifyAndSubmitProof()
    |
    +-- Step 1: certifyFile() / certifyHash()
    |       POST /api/proof → xProof API
    |       Returns: { id, hash, status, txHash }
    |
    +-- Step 2: submitProof()
    |       On-chain submit_proof(jobId, proofHash)
    |       → Validation Registry (MX-8004)
    |
    v
Two-layer proof:
  - xProof: immutable content hash + timestamp + explorer link
  - MX-8004: on-chain job proof for the agent economy

Auth modes

• API key: Authorization: Bearer pm_... (set via XPROOF_API_KEY env var)
• x402: HTTP 402 payment protocol -- USDC on Base, no account needed. Pass useX402: true + x402Payment header value.

x402 payment challenge

When calling without an API key and without a valid x402Payment, the xProof API returns HTTP 402 with a JSON body:

{
  "x402Version": 1,
  "accepts": [{ "scheme": "exact", "price": "50000", "network": "base", "payTo": "0x..." }]
}

The skill throws XProofPaymentRequired with the full payment details. The caller is responsible for completing the x402 payment and retrying with the x402Payment parameter (base64-encoded payment receipt).

Files changed

• src/skills/xproof_skills.ts -- new skill module
• src/skills/index.ts -- barrel export updated
• src/config.ts -- added CONFIG.XPROOF block (BASE_URL, API_KEY)
• .env.example -- added XPROOF_API_KEY, XPROOF_BASE_URL

Code conventions

• Follows existing patterns: Logger, CONFIG, JSDoc headers, exported types
• Routes env vars through CONFIG.XPROOF (not raw process.env)
• Imports submitProof from validation_skills.ts for the composite flow
• No new dependencies -- uses Node built-in crypto and native fetch