feat(ui): merge action and robust jump-to-task in worktrees overview

Add Merge entry to the worktrees overview context menu wiring the existing
MergeModalViewModel, replace fire-and-forget list selection with a
collection-change-aware JumpToTaskHelper, and propagate list renames to
visible task rows via a new ListUpdated event.

Harden worktree state changes: WorkerHub.SetWorktreeState now rejects
invalid transitions, WorktreeMaintenanceService only drops the DB row when
the on-disk worktree was actually removed, and Cleanup/Reset broadcast
WorktreeUpdated for affected tasks. SetWorktreeStateAsync returns the hub
error message so the modal can surface it.

Also: de-duplicate the worktrees overview modal opener, hook
OnParentTaskIdChanged to refresh IsDraft, fix MergeModal CanExecute
notifications, and add WorktreeStateHubTests for the transition rules.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Modals/WorktreesOverviewModalView.axaml

@@ -32,6 +32,10 @@
                       Command="{Binding $parent[Window].((vm:WorktreesOverviewModalViewModel)DataContext).JumpToTaskCommand}"
                       CommandParameter="{Binding}"/>
             <Separator/>
+            <MenuItem Header="Merge…"
+                      IsEnabled="{Binding IsActive}"
+                      Command="{Binding $parent[Window].((vm:WorktreesOverviewModalViewModel)DataContext).MergeCommand}"
+                      CommandParameter="{Binding}"/>
             <MenuItem Header="Discard"
                       IsEnabled="{Binding IsActive}"
                       Command="{Binding $parent[Window].((vm:WorktreesOverviewModalViewModel)DataContext).DiscardCommand}"