Tech debt: vendored pytest_shard is stale

`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).

Start by inspecting the vendored implementation under py/private/pytest_shard/ and compare its current behavior with the broader test driver roadmap. Determine whether the intended outcome is an upgrade or decoupling so users can provide their own sharding plugin; the work is done when the stale coupling is addressed and the chosen approach is documented.

Written by the indexing model from the issue text.