feat(ui): wire worktree overview modal entry points

Add list context-menu command (per-list mode) and Help menu entry (global mode) for the WorktreesOverviewModal; register VM and factory in DI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -69,6 +69,8 @@
                         Command="{Binding CheckForUpdatesCommand}"/>
               <MenuItem Header="Restart worker"
                         Command="{Binding RestartWorkerCommand}"/>
+              <MenuItem Header="Worktrees…"
+                        Command="{Binding OpenWorktreesOverviewGlobalCommand}"/>
               <MenuItem Header="About…" Command="{Binding OpenAboutCommand}"/>
             </MenuItem>
           </Menu>