docs(commands): make c4 skills notation-independent and level-consistent

Two audit fixes to c4-analyze and c4-diagram. Both treated draw.io as the only output. They now offer draw.io, Structurizr DSL, Mermaid (native C4 types), and PlantUML, with a headless fallback that emits a text notation instead of failing when drawio or a GUI is absent. Both also gained an abstraction-boundary section (a Container is a deployable unit, not a Docker container, and a Component isn't separately deployable) and a check that every element and relationship stays at the diagram's single C4 level.

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