This is the core of "never type a path from scratch": from the repo root,
the configured default path, and the parents of existing worktrees, derive
the ordered list of directories under which a new worktree could be placed.
Pure and unit-tested here; wired into the creation flow in a later commit.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
