Interactive C4 architecture diagrams (Simon Brown's model) for Claude Code. Five mode-driven workflows — Design (greenfield), Document-code (retro-doc from a repo), Document-prose (from README/ADR/spec), Review (critique an existing diagram), Update (evolve one) — accessed via /c4m:auto, /c4m:design, /c4m:code, /c4m:prose, /c4m:review, /c4m:update. Output is one Markdown document per C4 level with embedded Mermaid; format negotiable (Structurizr DSL, PlantUML) and destination negotiable (filesystem or MCP — Notion, Linear, Drive). Enforces the editorial invariants: technology mandatory on every Container/Component, intent-specific relationship labels, protocol on inter-container links, assumptions kept explicit.