Task: Add PlantUML recipe and extend codegen with puml scaffold

Two deliverables: (1) a recipe doc that walks a developer through adding a PlantUML diagram to any org-mode document — from writing the .puml source to exporting the PNG and embedding it in org-mode with correct figure metadata; (2) a compass add diagram <slug> sub-command that scaffolds a .puml file with the standard AGPL licence header, @startuml / @enduml, and the shared skin/style preamble, matching the convention of existing .puml files in the repository. Existing diagrams without the standard header are updated.

• doc/recipes/plantuml/how_do_i_add_a_plantuml_diagram.org exists (org-roam ID, v2 frontmatter) and covers: prerequisites, writing .puml source, running plantuml to export PNG, embedding via #+attr_html: / #+attr_latex: / #+caption: in an org-mode file.
• compass add diagram <slug> scaffolds <slug>.puml with the standard header; the skeleton compiles without errors under plantuml.
• All existing .puml files in the repo have the standard licence header.

(Transient implementation strategy. Written when work starts; distilled into the parent story's * Decisions and cleared when the task closes. Plans do not outlive their task.)