Add a Workbench tool

1. Create a session with explicit runtime bindings/state.

2. Implement WorkbenchToolContribution at the composition edge.

3. Reference workspace documents; do not copy or own them.

4. Supply primary views, local actions, status, and a summary.

5. Register through WorkbenchToolFactory or the host composition root.

The view selector is presentation, not session lifetime. The unshipped examples/python_tool_mode.py demonstrates exposure, not a Python render core.