chore(mise): point venv path to py/.venv

[Matt Perpick (clutchski)]

Summary

• Updates the mise venv path from venv to py/.venv to match the SDK workspace layout under py/

Test plan

• Verify mise install activates the virtualenv at py/.venv

🤖 Generated with Claude Code

fixes this warning when running mise install

     + wrapt==1.17.3
    uv sync --group dev
    warning: `VIRTUAL_ENV=/Users/matt/code/braintrustdata/braintrust-sdk-python/venv` does not match the project environment path `.venv` and will be ignored; use `--active` to target the active environment instead
    Resolved 299 packages in 3ms