feat: add agent-payment-x402 skill for autonomous agent payments

Adds a skill for x402 payment execution with MCP integration:
- Per-task and per-session spending controls
- Non-custodial wallet management (ERC-4337)
- Pairs with mcp-server-patterns and cost-aware-llm-pipeline skills
- Production reference: merged into NVIDIA NeMo Agent Toolkit (PR #17)
- npm package: agentwallet-sdk

skills/agent-payment-x402/SKILL.md

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+---
+name: agent-payment-x402
+description: Add x402 payment execution to AI agents — per-task budgets, spending controls, and non-custodial wallets via MCP tools. Use when agents need to pay for APIs, services, or other agents.
+origin: community
+---
+
+# Agent Payment Execution (x402)
+
+Enable AI agents to make autonomous payments with built-in spending controls. Uses the x402 HTTP payment protocol and MCP tools so agents can pay for external services, APIs, or other agents without custodial risk.
+
+## When to Use
+
+Use when: your agent needs to pay for an API call, purchase a service, settle with another agent, enforce per-task spending limits, or manage a non-custodial wallet. Pairs naturally with cost-aware-llm-pipeline and mcp-server-patterns skills.
+
+## Core Concepts
+
+### x402 Protocol
+x402 extends HTTP 402 (Payment Required) into a machine-negotiable flow. When a server returns `402`, the agent's payment tool automatically negotiates price, checks budget, signs a transaction, and retries — no human in the loop.
+
+### Spending Controls
+Every payment tool call enforces a `SpendingPolicy`:
+- **Per-task budget** — max spend for a single agent action
+- **Per-session budget** — cumulative limit across an entire session
+- **Allowlisted recipients** — restrict which addresses/services the agent can pay
+- **Rate limits** — max transactions per minute/hour
+
+### Non-Custodial Wallets
+Agents hold their own keys via ERC-4337 smart accounts. The orchestrator sets policy; the agent can only spend within bounds. No pooled funds, no custodial risk.
+
+## MCP Integration
+
+The payment layer exposes standard MCP tools that slot into any Claude Code or agent harness setup:
+
+```json
+{
+  "mcpServers": {
+    "agentpay": {
+      "command": "npx",
+      "args": ["-y", "agentwallet-sdk"]
+    }
+  }
+}
+```
+
+### Available Tools
+
+| Tool | Purpose |
+|------|---------|
+| `get_balance` | Check agent wallet balance |
+| `send_payment` | Send payment to address or ENS |
+| `check_spending` | Query remaining budget |
+| `set_policy` | Configure spending limits |
+| `list_transactions` | Audit trail of all payments |
+
+## Example: Pay-Per-API-Call Agent
+
+```typescript
+// In your CLAUDE.md or agent config:
+// 1. Add agentpay MCP server (see above)
+// 2. Set spending policy in your skill/hook:
+
+// Hook: pre-tool check
+if (toolName === "web_search" && apiCost > 0) {
+  const budget = await mcp.call("agentpay", "check_spending");
+  if (budget.remaining < apiCost) {
+    return { error: "Budget exceeded for this task" };
+  }
+}
+```
+
+## Best Practices
+
+- **Set budgets before delegation**: When spawning sub-agents, attach a SpendingPolicy. Never give an agent unlimited spend.
+- **Audit trails**: Use `list_transactions` in post-task hooks to log what was spent and why.
+- **Fail closed**: If the payment tool is unreachable, block the paid action — don't fall back to unmetered access.
+- **Pair with security-review**: Payment tools are high-privilege. Apply the same scrutiny as shell access.
+- **Test with testnets first**: Use Base Sepolia for development; switch to Base mainnet for production.
+
+## Production Reference
+
+- **npm**: [`agentwallet-sdk`](https://www.npmjs.com/package/agentwallet-sdk)
+- **Merged into NVIDIA NeMo Agent Toolkit**: [PR #17](https://github.com/NVIDIA/NeMo-Agent-Toolkit-Examples/pull/17) — x402 payment tool for NVIDIA's agent examples
+- **Protocol spec**: [x402.org](https://x402.org)