When nuking a polecat while your shell is cd'd into its worktree,
the shell becomes completely broken (even `echo hello` fails).
Add a check before deletion that detects if the current working
directory is inside the polecat's worktree. If so, return an error
with helpful instructions to cd elsewhere first.
The check runs ALWAYS, even with --force, because this is about
shell safety not data loss. You can't bypass it - just cd out first.
Fixes#942
212f08ad03