feat: Make shell configurable via config file

[ezynda3]

Summary

• Add ability to configure the shell used by the bash tool via the config file
• Default to $SHELL environment variable if not specified in config
• Fall back to /bin/bash if neither config nor environment variable is set
• Allow customizing shell arguments

This is needed because I use nushell as my default shell on NixOS and need to specify a very specific path for bash.

Test plan

• Verify that shell configuration works with different shells
• Verify that shell arguments can be customized
• Verify backward compatibility with existing behavior