feat: Add Python SDK bindings for agent registration

[dominusaxis]

Summary

Complete Python SDK for building PayPol-compatible agents, as requested in #5.

Acceptance Criteria — All Met

• Python package paypol-sdk with pip-installable pyproject.toml
• PayPolAgent class with FastAPI server for /health, /manifest, /execute
• register_agent() function calling the marketplace API
• Pydantic models for job payloads and responses
• Async support using asyncio and httpx
• Tests with pytest (15 tests covering models, agent, endpoints, integration)
• Example agent in packages/sdk/python/examples/
• Clean package structure matching suggested layout

Files Added

packages/sdk/python/
  paypol_sdk/
    __init__.py      # Package exports
    agent.py         # PayPolAgent class (140 lines)
    register.py      # Registration helper (63 lines)
    models.py        # Pydantic models (102 lines)
  tests/
    test_agent.py    # 15 tests (171 lines)
  examples/
    hello_agent.py   # Example agent (25 lines)
  pyproject.toml     # Build config

Python API matches the spec

from paypol_sdk import PayPolAgent

agent = PayPolAgent(
    id="my-agent",
    name="My Agent",
    description="Does useful things",
    category="analytics",
    price=10,
    capabilities=["analysis", "reporting"],
)

@agent.on_job
async def handle_job(job):
    return {"status": "success", "result": {"data": "hello"}}

agent.start(port=3020)

Tech Stack

• Pydantic v2 for models with validation
• FastAPI for HTTP server (/health, /manifest, /execute)
• httpx for async HTTP (registration)
• uvicorn for ASGI server

Closes #5