Resolve immutable snippet sources

[danielporterda]

What changed

• reads an immutable snippet source at the exact commit declared in its full GitHub blob reference
• returns the resolved commit and bytes
• enforces a configurable source-size limit

Why

This is the smallest production source-resolution path and does not involve pull requests, local checkouts, HTTP implementation, or extraction.

Validation

• 2 focused pytest tests
• Ruff
• mypy
• git diff --check