refreshWorktrees now writes Model.Worktrees in an
onUIThreadUnlessRepoChanged bounce. loadWorktrees becomes a pure loader
that returns the worktrees instead of writing them, since it's shared
with refreshBranches; refreshWorktrees bounces the result, and the
branches call site writes it directly for now (that write moves into
refreshBranches's own bounce when that scope is migrated).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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