✨ 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.

package.json

@@ -19,6 +19,7 @@
     "production": "npm run build && npm run canvas",
     "prepublishOnly": "npm run build",
     "setup": "node dist/index.js setup",
+    "update": "node dist/index.js update",
     "type-check": "npx tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",