feat: add export_pcb_pdf tool (kicad-cli, full layer-plot PDF)

New MCP tool `export_pcb_pdf` wrapping `kicad-cli pcb export pdf`: exposes
the full layer-plot surface (layer/common-layer lists, mirror,
refdes/value exclusion, border+title, soldermask subtraction, the four
DNP fab-layer modes, negative, B&W, theme, drill-shape, and the
single/separate/multipage output modes). Rich CLI sibling of the existing
SWIG export_pdf (left untouched). Reads the saved .kicad_pcb.

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

src/tools/registry.ts

@@ -65,6 +65,7 @@ export const toolCategories: ToolCategory[] = [
       "export_ipcd356",
       "export_gencad",
       "export_pos",
+      "export_pcb_pdf",
       "export_pdf",
       "export_svg",
       "export_3d",