✨ feat(cli): add interactive update command for skill and config updates (#10)

Existing users who update the npm package often forget to update the agent
skill files, leaving their AI agent working with stale workflow guidance
(sizing rules, color palettes, anti-patterns). The new `update` subcommand
solves this by detecting and updating all skill installations automatically.

🔧 Interactive update wizard:
- Scan all agent directories (Cursor, Claude Code, Codex CLI) across global
  and local scopes for existing excalidraw-skill installations
- Batch-update found installations with a single Y/n confirmation
- Offer to install the skill for detected agents that don't have it yet
- Optionally re-apply MCP config (defaults to no for non-breaking updates)

📝 Documentation:
- Rewrite README "Updating" section to lead with the interactive command
- Add collapsible example session showing the update flow
- Retain manual update methods as fallback for each installation path

🎯 Ensures skill files stay in sync with MCP tools across releases,
preventing the subtle drift where new tool capabilities exist but the
agent's workflow guidance doesn't reference them.

src/index.ts

@@ -2727,12 +2727,17 @@ if (isMainModule()) {
       process.stderr.write(`Setup failed: ${(error as Error).message}\n`);
       process.exit(1);
     });
+  } else if (arg === 'update') {
+    import('./setup.js').then(m => m.runUpdate()).catch(error => {
+      process.stderr.write(`Update failed: ${(error as Error).message}\n`);
+      process.exit(1);
+    });
   } else if (arg === '--help' || arg === '-h' || arg === '--version' || arg === '-v') {
     const pkgPath = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..', 'package.json');
     try {
       const pkg = JSON.parse(fs.readFileSync(pkgPath, 'utf-8'));
       if (arg === '--help' || arg === '-h') {
-        process.stdout.write(`${pkg.name} v${pkg.version}\n\nUsage:\n  mcp-excalidraw-local          Start MCP server (stdio transport)\n  mcp-excalidraw-local setup     Interactive setup wizard\n  mcp-excalidraw-local --help    Show this help\n  mcp-excalidraw-local --version Show version\n`);
+        process.stdout.write(`${pkg.name} v${pkg.version}\n\nUsage:\n  mcp-excalidraw-local          Start MCP server (stdio transport)\n  mcp-excalidraw-local setup     Interactive setup wizard\n  mcp-excalidraw-local update    Update agent skills and MCP config\n  mcp-excalidraw-local --help    Show this help\n  mcp-excalidraw-local --version Show version\n`);
       } else {
         process.stdout.write(`${pkg.version}\n`);
       }