Task dispatch computed baseBranch as the phase branch name but never
ensured it existed in the git clone. When phases weren't dispatched
through the PhaseDispatchManager (which creates branches), the
git worktree add failed with "fatal: invalid reference".
Now DefaultDispatchManager calls ensureBranch for both the initiative
and phase branches before spawning, matching what PhaseDispatchManager
already does.
50aea7e6f1