Publish the generated catalog at a stable raw JSON tap URL

Parent epic: #15

Depends on: #17

Why

DataDesigner needs one default NVIDIA tap URL that users can consume without GitHub API auth. Since the generated catalog is checked in at catalog/plugins.json, the default should be the unauthenticated raw GitHub URL for that file.

Default NVIDIA tap URL:

https://raw.githubusercontent.com/NVIDIA-NeMo/DataDesignerPlugins/main/catalog/plugins.json

The tap contract is transport-generic: any HTTP(S) URL or local path that resolves to raw schema v2 JSON is a tap. Human docs should link to the raw JSON URL; they are not the machine-readable catalog delivery path.

Implementation

• Add [tool.ddp.tap].catalog-url with the exact default URL above as part of #21.
• Keep catalog/plugins.json as the checked-in generated source of truth.
• Do not add a docs/Pages publishing requirement for the machine-readable tap.
• Document freshness semantics:
  • .../main/catalog/plugins.json tracks accepted main and is mutable.
  • tag-pinned raw URLs are immutable snapshots, for example https://raw.githubusercontent.com/NVIDIA-NeMo/DataDesignerPlugins/<tag-or-sha>/catalog/plugins.json.
  • release assets can be added later only if we need a separate snapshot distribution mechanism.
• Add or preserve CI coverage through make check-catalog plus schema v2 tests from #17.
• Update README/docs to identify the default URL and explain that external taps may use any unauthenticated raw JSON endpoint or local file path.

Acceptance criteria

• The default NVIDIA tap URL is exactly documented as https://raw.githubusercontent.com/NVIDIA-NeMo/DataDesignerPlugins/main/catalog/plugins.json.
• catalog/plugins.json remains generated and checked in.
• CI validates the checked-in catalog shape and freshness through make check-catalog and schema v2 tests.
• No issue, docs page, or generated workflow requires GitHub Pages to expose the machine-readable catalog.
• DataDesigner can consume the URL without GitHub API authentication.

Dependencies

• Depends on: #17.
• Blocks: #23, #25, and DataDesigner default tap configuration.

Start by reading dependencies #17 and #21, then inspect catalog/plugins.json, the tap configuration, README/docs, and the make check-catalog target. Run make check-catalog and the schema v2 tests from #17. Done means the exact raw URL is documented, the checked-in catalog remains validated, and no GitHub Pages requirement is introduced.