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)

src/cli/init.ts

@@ -773,7 +773,7 @@ export function buildManifest(
   const { keywords, license, repository } = optionalFields;
 
   return {
-    manifest_version: "0.1",
+    $schema: "https://static.modelcontextprotocol.io/schemas/2025-08-26/mcpb.manifest.schema.json",
     name,
     ...(displayName && displayName !== name
       ? { display_name: displayName }