feat(mcp): add docs query server

[pcharbon70]

Summary

• add a tools-only MCP docs server core with search_docs, get_doc, and list_sections
• expose the same read-only docs server over mix mcp.docs and POST /mcp/docs
• cover the MCP core, stdio transport, HTTP transport, and docs tool behavior with tests

Testing

• mix compile --warnings-as-errors
• mix credo --strict
• mix test
• mix dialyzer

Closes #49