Remove template plugin from public catalog

[eric-tramel]

What

Removes data-designer-template from the checked-in public catalog at catalog/plugins.json.

The template package remains in the repository as the reference implementation and scaffold example, but it is no longer advertised as an installable package through the published NVIDIA catalog.

Why

Now that catalog entries are registered explicitly for first release, the reference template should not stay in the public catalog. It is documentation and authoring material, not a package we intend users to discover and install from the catalog.

Usage

No user-facing command changes. After this deploys, the published catalog should include only release-intended packages:

data-designer-github
data-designer-retrieval-sdg

How

• Removed the data-designer-template package object from catalog/plugins.json.
• Added a checked-in catalog assertion that the template package is excluded.
• Updated release-validation coverage so the template plugin is expected not to be publicly cataloged.
• Changed the release Makefile usage comment to use a non-template package name.

Validation

• make lint
• make test-devtools
• make check
• make docs

[johnnygreco]

Reviewed the public catalog update, release-validation test change, and Makefile example. Local validation is green (make lint, make test-devtools, make check, make docs), and the PR checks are passing. Looks good to me.