feat: add export_gerbers tool (kicad-cli, full Plot option set)

New MCP tool `export_gerbers` wrapping `kicad-cli pcb export gerbers`,
exposing the complete Plot-dialog option surface that the existing
SWIG-based export_gerber lacks: layer + common-layer lists, X2 on/off,
netlist attributes, soldermask subtraction, aperture macros, the three
DNP fab-layer modes, refdes/value exclusion, border+title, drill-file
origin, coordinate precision (5/6), Protel vs KiCad extensions, and
--board-plot-params to reuse the board's stored plot settings.

Implemented as a self-contained interface handler (_handle_export_gerbers)
that resolves the board path (param or current board) and shells out to
kicad-cli, mirroring the _handle_export_netlist pattern — no dependence
on a live SWIG board, since the KiCad IPC API exposes no plot/export
endpoints. Reads the saved .kicad_pcb on disk.

Wiring: dispatch map + src/tools/export.ts registration + export
category in registry.ts.

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

src/tools/registry.ts

@@ -58,6 +58,7 @@ export const toolCategories: ToolCategory[] = [
     description: "File export for fabrication and documentation: Gerber, PDF, BOM, 3D models",
     tools: [
       "export_gerber",
+      "export_gerbers",
       "export_pdf",
       "export_svg",
       "export_3d",