Load snippet repository registry

[danielporterda]

What changed

Adds a typed loader for the checked-in snippet repository registry.

It reads repository name, canonical GitHub URL, default branch, and visibility; returns entries in deterministic order; and rejects unreadable JSON, missing fields, non-canonical URLs, empty branches, and unsupported visibility values.

This PR does not parse source references or connect the registry to <Snippet> declarations.

Why

Loading and checking the registry's own shape is a separate concern from deciding whether a particular snippet reference belongs to an allowlisted repository.

Validation

• python -m pytest tests/test_snippet_repository_registry.py (7 passed)
• python -m ruff check scripts/snippets/registry.py tests/test_snippet_repository_registry.py
• python -m mypy scripts/snippets/registry.py
• 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.