Add repo-level tap metadata config for ddp tooling

Parent epic: #15

Depends on: #16

Why

DDPlugins should be both NVIDIA's curated first-party tap and the reference implementation for external tap repositories. Tooling should not hardcode NVIDIA-specific values in scaffold output or catalog generation. Those values should live in one repo-level tap config.

Proposed config

Add this table to the root pyproject.toml:

[tool.ddp.tap]
catalog-url = "https://raw.githubusercontent.com/NVIDIA-NeMo/DataDesignerPlugins/main/catalog/plugins.json"
repository-url = "https://github.com/NVIDIA-NeMo/DataDesignerPlugins"
repository-git-url = "https://github.com/NVIDIA-NeMo/DataDesignerPlugins.git"
docs-base-url = "https://nvidia-nemo.github.io/DataDesignerPlugins/"
package-prefix = "data-designer-"
default-source = "pypi"
release-ref-template = "{package}/v{version}"
default-data-designer-requirement = "data-designer>=0.5.7"
author-name = "NVIDIA Corporation"

Field semantics:

• catalog-url: default tap URL advertised for this repo.
• repository-url: human-facing repository URL used in package metadata and docs.
• repository-git-url: clone/install URL used for schema v2 git source objects.
• docs-base-url: base URL used to derive docs.url; plugin docs URLs are {docs-base-url.rstrip('/')}/plugins/{docs_slug}/.
• package-prefix: expected plugin package prefix for scaffolding and validation.
• default-source: one of pypi, git, or path; NVIDIA default is pypi.
• release-ref-template: Python .format(package=..., version=...) template for per-plugin tags.
• default-data-designer-requirement: dependency string used by new scaffolds.
• author-name: default author used by new scaffolds.

Implementation

• Add a small ddp config loader, for example ddp.tap_config, that reads [tool.ddp.tap] from the repo root.
• Treat this table as required for this repo. If missing, fail with a clear error that names the missing table.
• Use the config in catalog generation, source metadata generation, scaffold generation, and docs examples.
• Replace the scaffold's stale hardcoded data-designer>=0.5.3 default with default-data-designer-requirement.
• Preserve root zensical.toml as the docs build config, but do not use it as the only source of tap metadata.
• Add tests with a temporary repo config proving that non-NVIDIA URLs/authors are substituted into generated output.

Acceptance criteria

• Root pyproject.toml contains the [tool.ddp.tap] table above.
• ddp tooling reads tap metadata through one shared loader.
• New scaffolded plugins use the configured repository URL, docs URL base, author, and data-designer dependency.
• Catalog source/docs metadata uses the configured URLs and source policy.
• A temp external-style tap config produces generated files without NVIDIA URLs unless those URLs are configured.
• Missing or malformed config has deterministic tests and clear error messages.

Dependencies

• Depends on: #16.
• Blocks: #17, #18, #22, and external tap authoring docs in #25.

Start with the root pyproject.toml and the existing ddp tooling, then add the shared ddp.tap_config loader described in the issue. Trace catalog, source metadata, scaffold, and docs-example generation to replace hardcoded values, and add temporary-repository tests covering substituted metadata plus missing or malformed configuration errors.