Add MCP server + PyPI packaging and release automation

MCP server (okfgen-mcp) exposes generate/search/ask/validate/visualize/
list_source_types as Model Context Protocol tools, so Claude Desktop, Claude
Code, Cursor, and any MCP client can produce and reason over OKF bundles.
Module stays importable without the optional `mcp` dependency.

Packaging: PyPI-ready metadata (classifiers, URLs, keywords), an [mcp] extra,
the okfgen-mcp entry point, a Trusted-Publishing release workflow, and
PUBLISHING.md. Verified: build + twine check pass, wheel installs clean in a
fresh venv, both console scripts run, and the MCP server lists all six tools.

README: "Use it inside your AI agent (MCP)" section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>