Generate Archify cards from registry descriptions for model diagrams

An architecture diagram inside a model that defines no cards gets one card
per drawn shared element with a description, as the extension does. Test
covers the resolved JSON; formats.md and SKILL.md describe the rule.

references/formats.md

@@ -100,3 +100,7 @@ For each diagram file under the root (and a single file outside a model):
 5. Hand the result to `archify validate <kind> <json> --json` (or `render`).
 
 `task model:resolve -- --file=… --out=x.json` writes step 4's result.
+
+## Cards under a model diagram
+
+Archify shows `cards` (a coloured dot, a title, a few lines) below a diagram. An architecture diagram inside a model that draws shared elements and defines no `cards` of its own gets one card per drawn element that has a `description` in the registry, in drawing order, with the dot colour of the element's type (frontend cyan, backend emerald, database violet, cloud amber, security rose, messagebus orange, external slate). `model:resolve` and the extension both add them; write your own `cards:` in the diagram to replace them.