support different base branches better

default is origin/master. consider using origin/main and making that switch easier than toggling an environmentvariable (maybe in-cfg or something)

Start by locating the configuration and environment-variable handling for the default origin/master base branch. Check how the tool selects a base branch, then determine how origin/main and an easier configuration option should fit that path. Done means users can choose the base branch without toggling an environment variable.

Written by the indexing model from the issue text.