fix(devshell): isolate uv from Nix Python packages

[jcardozo-eth]

Closes #39.

Summary

Before this change, Nix development shells exposed Python packages propagated by Nix’s pre-commit package through PYTHONPATH. Those entries took precedence over uv’s project environment, so uv run pytest could launch pytest from .venv while importing _pytest from the Nix store.

The shared development-shell shellHook now unsets the inherited PYTHONPATH before installing Git hooks. Project commands therefore resolve dependencies from .venv, while Nix’s wrapped pre-commit executable continues to install and run normally.

This fixes the environment mismatch exposed by #38 without changing CI commands or project dependencies.

Test plan

• In py310 through py314, verify PYTHONPATH is unset, pytest imports from the matching .venv, no Nix-store site-packages entries appear in sys.path, and all 78 tests pass.
• Run pre-commit validate-config .pre-commit-config.yaml and pre-commit run --all-files --show-diff-on-failure.
• Run nix flake check --no-build.
• Confirm CI passes for lint, matrix derivation, and every Python test job.
• After this PR is created, rebase chore: bump pytest from 9.0.3 to 9.1.1 #38 onto this branch and confirm its pytest 9.1.1 matrix passes.

Checklist

• Tests green locally (uv run pytest)
• pre-commit run --all-files exits successfully
• nix flake check --no-build exits successfully
• PR CI is green
• chore: bump pytest from 9.0.3 to 9.1.1 #38 is green after rebasing onto this branch