Git worktree names are stored in .git/worktrees/<name> and cannot
contain slashes. The code was using branch names like "feature/slug"
or "rfc/name" as worktree names, which git2 rejects silently.
Now the worktree name is derived from the path's directory name (the
slug), which is always a simple identifier without slashes.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>