feat: add export_pcb_dxf tool (kicad-cli, full layer-plot DXF)

New MCP tool `export_pcb_dxf` wrapping `kicad-cli pcb export dxf`: exposes
the full layer-plot surface (layer/common-layer lists, refdes/value
exclusion, soldermask subtraction, use-contours, use-drill-origin,
border+title, output-units, the four DNP fab-layer modes, drill-shape, and
the single/multi output modes). Reads the saved .kicad_pcb.

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

src/tools/registry.ts

@@ -67,6 +67,7 @@ export const toolCategories: ToolCategory[] = [
       "export_pos",
       "export_pcb_pdf",
       "export_pcb_svg",
+      "export_pcb_dxf",
       "export_pdf",
       "export_svg",
       "export_3d",