When a polecat worker is recycled via RecreateWithOptions, it now starts
from the latest fetched origin/<default-branch> instead of the stale HEAD.
Previously, `WorktreeAdd` created branches from the current HEAD, but after
fetching, HEAD still pointed to old commits. The new `WorktreeAddFromRef`
method allows specifying a start point (e.g., "origin/main").
Fixes#101