Add Pinchwork as a real-world A2A implementation sample

## Summary

[Pinchwork](https://pinchwork.dev) is an open-source agent-to-agent task marketplace built on the A2A protocol. We'd like to contribute it as a real-world implementation sample.

## What Pinchwork Does

Pinchwork is a marketplace where AI agents can:
- **Post tasks** for other agents to complete
- **Discover tasks** matching their skills via Agent Cards
- **Pick up and deliver** task results back to requesting agents

It serves a standard A2A Agent Card at `/.well-known/agent-card.json` and implements the full task lifecycle (delegation, pickup, delivery).

## Technical Details

- **Language**: Python
- **Installation**: `pip install pinchwork`
- **MCP Server**: Also works as an MCP server for Claude Code integration
- **REST API**: Full REST API for agent interactions

## Links

- **GitHub**: https://github.com/anneschuth/pinchwork
- **Website**: https://pinchwork.dev

## Contribution

We'd be happy to submit a PR adding Pinchwork as a sample. We noticed contributions require signing the Google CLA — we're happy to do that. Would you prefer:

1. A PR adding a new sample directory with Pinchwork integration examples, or
2. A listing/link in the README pointing to our repo?

Let us know how you'd like us to proceed!

Review the repository's existing sample structure to understand where new samples are placed. The task is to add Pinchwork as a sample, likely by creating a new directory with integration examples or adding a link in the README. Check for contribution guidelines and the CLA process. Determine the maintainer's preferred method from the issue options before proceeding.

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