Tech debt: vendored pytest_shard is stale

[arrdem]

py/private/pytest_shard/ contains a vendored sharding implementation for pytest. It appears stale and tightly coupled.

We should either:

• Upgrade it to a maintained version
• Adopt a pattern that lets users bring their own sharding plugin (e.g., pytest-split, pytest-xdist)

This would align with the broader effort to decouple test framework specifics from core rules (see tracking issue for test driver roadmap).