feat: add gears workbench as default addon

Add kindred/gears as a submodule under mods/gears. The gears workbench
provides 13 parametric gear commands (involute, cycloid, bevel, worm,
crown, timing, lantern, hypocycloid gears + connectors).

Integration:
- Kindred package.xml with load_priority 40 (sdk -> solver -> gears -> silo)
- Init.py adds repo root to sys.path for pygears + namespace package
- InitGui.py imports upstream GearWorkbench and injects gear commands
  into partdesign.body and partdesign.feature editing contexts
- context.py kcsdk routing already complete from #351

.gitmodules

@@ -18,3 +18,6 @@
 	path = mods/solver
 	url = https://git.kindred-systems.com/kindred/solver.git
 	branch = main
+[submodule "mods/gears"]
+	path = mods/gears
+	url = https://git.kindred-systems.com/kindred/gears.git