Move package.xml from freecad/ to mod root following the standard
addon layout used by sdk, gears, datums, and solver.
Add root Init.py (sys.path setup for silo-client) and InitGui.py
that consolidates all Silo GUI initialization:
- SiloWorkbench class and registration
- Silo overlay context registration
- Document observer for .kc tree building
- Auth and activity dock panels via kindred_sdk
- First-start settings check
- Start page override
- kindred:// URL handler
This decouples Silo initialization from src/Mod/Create/InitGui.py,
which previously held five Silo-specific deferred setup functions.
