Proposal: Add agent402 integration sample — x402 buy + sell side

Agent402 is an open-source x402 + MCP server with 1,323 pay-per-call web tools for AI agents, plus `agent402-tollbooth` for pay-per-crawl on the sell side. Both speak vanilla x402, so an AgentCore-hosted Strands agent works end-to-end with no protocol bridging.

Happy to PR a small `03-integrations/agent402/` folder containing:

1. **Buy-side**: a Strands agent calling Agent402 tools via the published [`agent402-strands`](https://www.npmjs.com/package/agent402-strands) adapter (proof-of-work free tier; AgentCore Payments + CDP signs for wallet-only tools). Working code: https://github.com/MikeyPetrillo/Agent402/tree/main/examples/agentcore

2. **Sell-side**: a reverse-flow demo where a Strands agent pays a self-hostable tollbooth gate. Working code: https://github.com/MikeyPetrillo/Agent402/tree/main/examples/agentcore-tollbooth

3. **README** pointing at the 5-minute integration guide: https://github.com/MikeyPetrillo/Agent402/wiki/AWS-Bedrock-AgentCore

This closes the loop for the AgentCore Payments demo (`aws-samples/sample-agentcore-cloudfront-x402-payments`) by giving it a real buy-side counterparty + an open-source sell-side gate.

Will follow this issue with a PR once you confirm the folder location.

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**Links:**
- Repo: https://github.com/MikeyPetrillo/Agent402
- Live instance: https://agent402.tools
- MCP endpoint: https://agent402.tools/mcp
- x402 protocol: https://x402.org

Review the existing 03-integrations/ layout and the linked Agent402 AgentCore examples first. Confirm the folder location, then add the proposed buy-side and sell-side samples plus a README linking to the integration guide. Done means both flows are represented in the new folder and the README points to the documented setup.

Written by the indexing model from the issue text.