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>

runtime/mcp/EXCALIDRAW.json

@@ -0,0 +1,7 @@
+{
+  "name": "excalidraw",
+  "launch": "${MOSAIC_TOOLS}/excalidraw/launch.sh",
+  "enabled": true,
+  "required": false,
+  "description": "Headless .excalidraw → SVG export and diagram generation via @excalidraw/excalidraw"
+}