Add snippet repository registry

[danielporterda]

What changed

Adds the allowlisted repository registry for release-aware snippets. Each entry records only the repository's canonical GitHub URL, default branch, and visibility.

This PR intentionally adds no loader, parser, validation, release policy, or runtime behavior. Those will be independently reviewable layers above it.

Why

The snippet machinery needs one explicit source of repository identity before it can safely interpret inline source references. Keeping this data-only layer separate makes the allowlist and its scope reviewable without coupling it to syntax or validation decisions.

Validation

• jq empty config/snippet-repositories.json
• git diff --check

[danielporterda]

Closing this draft because the inline release-aware snippet stack is being retired. The branch remains available if its implementation is needed later.