.archify files are Archify JSON diagram sources. They open as JSON,
preview with the same commands, report validation problems, and get
completion from a schema that dispatches on diagram_type. Adds a
hello.archify example and tests for detection, the dispatching schema
and the VS Code behaviour.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
