ZTools no longer registers as a standalone workbench. Commands are now
injected into native workbench toolbars (PartDesign, Assembly,
Spreadsheet) via _ZToolsManipulator (WorkbenchManipulator) and the
EditingContextResolver system (injectEditingCommands).
- Remove ZToolsWorkbench class and Gui.addWorkbench() call
- Deferred registration via QTimer.singleShot(2000ms)
- Expand manipulator to cover Assembly and Spreadsheet toolbars
- Update package.xml description
