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>

examples/hello.archify

@@ -0,0 +1,17 @@
+{
+  "schema_version": 1,
+  "diagram_type": "architecture",
+  "meta": {
+    "title": "Hello Archify",
+    "output": "hello.html"
+  },
+  "components": [
+    { "id": "browser", "type": "frontend", "label": "Browser", "pos": [40, 120], "size": [140, 60] },
+    { "id": "api", "type": "backend", "label": "API", "pos": [260, 120], "size": [140, 60] },
+    { "id": "db", "type": "database", "label": "PostgreSQL", "pos": [480, 120], "size": [140, 60] }
+  ],
+  "connections": [
+    { "from": "browser", "to": "api", "label": "HTTPS" },
+    { "from": "api", "to": "db", "label": "SQL" }
+  ]
+}