Support .archify diagram files (0.2.0)

.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>

src/preview.ts

@@ -92,7 +92,10 @@ class DiagramPreview implements vscode.Disposable {
     }
     const source = readDiagramSource(text, this.uri.path);
     if (!source) {
-      this.showMessage('Not an Archify diagram', 'The file has no "diagram_type" of architecture, workflow, sequence, dataflow or lifecycle.');
+      this.showMessage(
+        'Nothing to show yet',
+        'Set "diagram_type" to architecture, workflow, sequence, dataflow or lifecycle, and make sure the file is valid JSON.',
+      );
       return;
     }