Create and initialize venv with agentstack init

[tcdent]

We currently leave creation and activation of the project's venv up to the user.

When initializing a project from a template, tools are added to the project. If the tool has any dependencies, as is the case with #76 they'll be installed in the user's global packages (or the currently active venv).

In the interest of making this as straightforward as possible, we should manage venvs for the user.