Add Archify | New Model… with three C4 examples and Open Model in the Explorer menu

- templates/models: C4 - Standard Web App, C4 - E-Shop, C4 - Event-Based
  Integration; each a complete model (main.archify, registry, context,
  containers, components) that validates
- archify.newModel: pick an example and a folder name, copy it, name the
  model after the folder, open the model page
- Explorer menu: New Model… on folders, Open Model on .archify and
  .archify-registry files; File → New File… lists New Model…
- unit test validates every example end to end; integration test runs the
  command; README, CHANGELOG, docs/model.md

docs/model.md

@@ -80,3 +80,7 @@ Registry `diagram` links and component `diagram` links are the same mechanism; t
 ## Outside a model
 
 A diagram with `ref` components outside any model (no `main.archify` above it) reports "no registry found" and shows the component by its ref name with type `external`, so the picture still renders.
+
+## Starting a model
+
+*Archify | New Model…* (right-click a folder, *File → New File…*, Command Palette) offers three complete examples: **C4 - Standard Web App**, **C4 - E-Shop** and **C4 - Event-Based Integration**. Pick one and a folder name; the example is copied into that folder with `model.id`, `model.name` and the overview title taken from the folder name (an example kept under its own name is copied as is). Each example has `registry/{people,systems,containers}.archify-registry` and diagrams under `0100-context`, `0200-containers` and `0300-components`; every registry element is used by at least one diagram and every diagram validates.