Add ARCHITECTURE.md documenting the Python plugin gRPC system including:
- High-level overview and ASCII system architecture diagram
- Component layer documentation (Protocol, Go Infrastructure, Python SDK)
- Process lifecycle (loading, environment variables, shutdown)
- Communication flow diagrams (hooks, API calls, ServeHTTP streaming)
- Key design decisions (embedded AppError, 64KB chunks, APIServerRegistrar)
- Complete file reference table mapping functionality to source files
- Extension guide for adding new API methods and hooks
This documentation enables internal Mattermost engineers to understand,
debug, and extend the Python plugin infrastructure.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
