Ephemeral environments for Julia+Python packages

I was thinking it would be nice to have something like `uv run --with numpy python script.py` for running things in ephemeral environments. I see my coding agents like using this sort of thing for quickly answering questions.

Perhaps a `juliapkg run` command?

```bash
uv run --with juliacall \
juliapkg run --pkg Statistics --pkg LinearAlgebra python script.py
```

Posting it in this repo rather than juliapkg as the specific home tool is part of the discussion too.

The issue names no existing files or tests. Start by reviewing the repository's current command-line and package-management entry points, then compare the proposed `juliapkg run` workflow with `uv run --with`; done requires an agreed tool location and specified behavior for ephemeral Julia/Python environments.

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