feat(schematic): hierarchical sheet insertion and subsheet scaffolding

Adds add_hierarchical_sheet (insert a sheet symbol referencing a child
.kicad_sch, with sheet_instances + fixed component instance paths) and
create_hierarchical_subsheet (create the child file + wire it into the parent
in one call). Upstream has add_sheet_pin / add_schematic_hierarchical_label
but no way to create a sheet or stand up a child sheet, so hierarchical
designs can't be built through the MCP server today.

- python/commands/schematic_hierarchy.py: SchematicHierarchyCommands
- src/tools/schematic-hierarchy.ts + registry 'schematic_hierarchy' category
- python/kicad_interface.py: import + instantiate + dispatch routes
- tests/test_schematic_hierarchy.py: 5 unit/integration tests on real .kicad_sch

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/tools/registry.ts

@@ -123,6 +123,11 @@ export const toolCategories: ToolCategory[] = [
     description: "Read a symbol's pins straight from the library (no schematic needed)",
     tools: ["list_symbol_pins", "batch_list_symbol_pins"],
   },
+  {
+    name: "schematic_hierarchy",
+    description: "Hierarchical schematic sheets: insert a sheet, scaffold a sub-sheet",
+    tools: ["add_hierarchical_sheet", "create_hierarchical_subsheet"],
+  },
   {
     name: "routing",
     description: "Advanced routing operations: vias, copper pours",