python: environment "health check"

[austin3dickey]

A new command that would check everything's in order to start a new Python session aligned with our recommendations. Some ideas:

• CPython 3.9-3.14 is installed
  • if not, download/use uv to install it
• you're in a workspace
  • if not, open or create one
• your workspace has a virtual environment
  • if not, create one