feat: add export_gencad tool (kicad-cli, GenCAD assembly/test format)

New MCP tool `export_gencad` wrapping `kicad-cli pcb export gencad`:
exposes define-var plus the flip-bottom-pads, unique-pins,
unique-footprints, use-drill-origin and store-origin-coord flags.
Self-contained interface handler reading the saved .kicad_pcb.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/tools/registry.ts

@@ -63,6 +63,7 @@ export const toolCategories: ToolCategory[] = [
       "export_ipc2581",
       "export_odb",
       "export_ipcd356",
+      "export_gencad",
       "export_pdf",
       "export_svg",
       "export_3d",