feat: replace manifest_version with $schema field

- Replace manifest_version with $schema field pointing to official schema URL
- Support backward compatibility with legacy dxt_version field
- Update all examples and tests to use new $schema field
- Mark dxt_version as deprecated but still functional
- Require either $schema or dxt_version (at least one must be present)

test/invalid-manifest.json

@@ -1,5 +1,5 @@
 {
-  "manifest_version": "1.0",
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-08-26/mcpb.manifest.schema.json",
   "name": "test-extension",
   "version": "1.0.0",
   "description": "A test DXT extension",