FR: Allow using environment variables (`${SHPOOL_SESSION_NAME}`) in `default_dir` config option

AI Policy Ack
Please read the AI policy in HACKING.md and ack that you've done so here.
Ack.
What new feature do you want?

I want to be able to use environment variables inside the default_dir setting, e.g.

default_dir = "/home/${USER}/scripts"

The main use case is to use the $SHPOOL_SESSION_NAME variable.

Why this is useful?
On my remote machine, I have a few different repositories that live under a common root (Think /home/$USER/coding/$repo). Using the local shell function approach, I can use work $repo to shpool into a re-usable session for that project. However, if that session does not exist, it currently starts at ~. I want to be able to set

default_dir = "/home/${USER}/coding/${SHPOOL_SESSION_NAME}"

in my settings so that newly-created sessions already start in the right directory.

Read HACKING.md first and trace how the default_dir configuration is parsed and used when creating a session. Verify the behavior with the provided USER and SHPOOL_SESSION_NAME examples; done means newly created sessions start in the expanded directory while existing behavior remains intact.