Check Python virtual environment exists

[vidya381]

When Python is detected, check if a virtual environment is set up. People often clone a Python project and forget to create one.

Check for .venv or venv directory in the project root. Fail with a hint to run python3 -m venv .venv if neither exists.

Add internal/check/venv.go, register in registry.go under stack.Python.

[vidya381]

Covered by #23 — the Python deps check already verifies that .venv or venv exists.