feat: integrate excalidraw MCP into bootstrap and runtime setup

- install.sh: run mosaic-ensure-excalidraw post-install (non-fatal)
- runtime-setup.ts: configure excalidraw MCP during wizard setup
- bin/mosaic-ensure-excalidraw: install deps + register MCP with Claude
- runtime/mcp/EXCALIDRAW.json: MCP server config template
- tools/excalidraw/: headless .excalidraw → SVG export server

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/excalidraw/stubs/laser-pointer.mjs

@@ -0,0 +1,7 @@
+/**
+ * Stub for @excalidraw/laser-pointer
+ * The real package uses a Parcel bundle format that Node.js ESM can't consume.
+ * For headless SVG export, the laser pointer feature is not needed.
+ */
+export class LaserPointer {}
+export default { LaserPointer };