Plugin-based MicroPython software architecture diagram generation phase. Used after receiving the optional_next_phases selection from upy-generate-plugin success, reads the generated firmware and project-manifest.json, generates docs/diagram.json, validates against diagram.schema.json, renders architecture/flowchart/data_flow as md/svg/png/html, and outputs a phase_complete with session/checkpoint/artifact manifest; compatible with plugin protocol invocation and local skill direct testing, does not overwrite the original upy-diagram.
