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>

install.sh

@@ -177,6 +177,12 @@ else
   fi
 fi
 
+if "$TARGET_DIR/bin/mosaic-ensure-excalidraw" >/dev/null 2>&1; then
+  ok "excalidraw MCP configured"
+else
+  warn "excalidraw MCP setup failed (non-fatal) — run 'mosaic-ensure-excalidraw' to retry"
+fi
+
 if [[ "${MOSAIC_SKIP_SKILLS_SYNC:-0}" == "1" ]]; then
   ok "Skills sync skipped (MOSAIC_SKIP_SKILLS_SYNC=1)"
 else