No built in way to override new-pane actions right now, but `cmux --help` can automate all parts of cmux.
So you can make your own script that can make new panels/workspaces and just invoke it from the terminal:
git worktree add -b my-branch ../repo-my-branch
ws=$(cmux new-workspace 2>&1 | awk '{print $2}')
cmux send --workspace "$ws" "cd ../repo-my-branch && claude"
cmux send-key --workspace "$ws" Enter
I think we should make this easier though, open to suggestions!