Publish agentspec Python SDK to PyPI

[skokaina]

Problem

Python users expect `pip install agentspec`. Currently the Python SDK (`packages/sdk-python/`) has a valid `pyproject.toml` but is not published to PyPI. The `sdk-langgraph` package (`packages/sdk-langgraph/`) is in the same situation.

For a launch targeting both TypeScript and Python agent developers, `pip install` must work.

Scope

1. agentspec — `packages/sdk-python/` → PyPI as `agentspec`
2. agentspec-langgraph — `packages/sdk-langgraph/` → PyPI as `agentspec-langgraph`

Tasks

• Verify `pyproject.toml` metadata (name, version, description, license, URLs)
• Add PyPI Trusted Publishing to GitHub Actions (or use API token)
• Add `publish-python.yml` workflow triggered on release/tag
• Test install from PyPI: `pip install agentspec agentspec-langgraph`
• Update README and docs with `pip install` instructions alongside `npm install`

Acceptance criteria

• `pip install agentspec` works from PyPI
• `pip install agentspec-langgraph` works from PyPI
• Versions match the npm SDK version