feat: publish pmmcp package to PyPI

[tallpsmith]

Summary

Publish the pmmcp package to PyPI so users can install it via pip install pmmcp or uv tool install pmmcp / uvx pmmcp without cloning the repo.

Acceptance Criteria

• pyproject.toml metadata is complete (description, classifiers, URLs, license)
• GitHub Actions workflow publishes to PyPI on version tag (using trusted publishing or API token secret)
• README Installation section updated with working uv tool install pmmcp and uvx instructions
• uvx pmmcp --pmproxy-url http://host:44322 works end-to-end

Context

The README previously documented this as "Option B" with a uvx MCP config snippet, but the package was never published. Users currently must build from source.