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>

src/stages/runtime-setup.ts

@@ -1,3 +1,7 @@
+import { spawnSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { join } from 'node:path';
+import { homedir } from 'node:os';
 import type { WizardPrompter } from '../prompter/interface.js';
 import type { WizardState, RuntimeName } from '../types.js';
 import { detectRuntime, type RuntimeInfo } from '../runtime/detector.js';
@@ -66,5 +70,20 @@ export async function runtimeSetupStage(
         `MCP setup failed: ${err instanceof Error ? err.message : String(err)}. Run 'mosaic seq fix' later.`,
       );
     }
+
+    // Configure excalidraw MCP (non-fatal — optional tool)
+    const mosaicHome = process.env['MOSAIC_HOME'] ?? join(homedir(), '.config', 'mosaic');
+    const ensureExcalidraw = join(mosaicHome, 'bin', 'mosaic-ensure-excalidraw');
+    if (existsSync(ensureExcalidraw)) {
+      const spin3 = p.spinner();
+      spin3.update('Configuring excalidraw MCP...');
+      const res = spawnSync(ensureExcalidraw, [], { encoding: 'utf8' });
+      if (res.status === 0) {
+        spin3.stop('excalidraw MCP configured');
+      } else {
+        spin3.stop('excalidraw MCP setup failed (non-fatal)');
+        p.warn("Run 'mosaic-ensure-excalidraw' manually if needed.");
+      }
+    }
   }
 }