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/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "excalidraw-mcp",
+  "version": "1.0.0",
+  "type": "module",
+  "private": true,
+  "dependencies": {
+    "@modelcontextprotocol/sdk": "^1.12.0",
+    "@excalidraw/excalidraw": "^0.18.0",
+    "jsdom": "^25.0.1"
+  }
+}