Allow setting default session directory via environment variable

[jamestut]

Background

The session directory is always computed from $HOME (~/.cache/atch/). This will be problematic when:

• Containers share a home directory bind mount — multiple containers clobber each other's sockets and logs.
• NFS-mounted centralised home — machines on the network see stale sockets from other hosts, causing connection failures or phantom sessions.

Solution

I'm adding ATCH_SESSION_DIR environment variable at get_session_dir() to allow user to override the default session directory location.