fix: Add bin field to root package.json for npx execution

- Points to installer CLI at tools/installer/bin/bmad.js
- Enables npx github:bmadcode/BMAD-METHOD#v4-alpha to work

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -3,7 +3,10 @@
   "version": "4.0.0",
   "description": "Breakthrough Method of Agile AI-driven Development",
   "main": "tools/cli.js",
-  "bin": {},
+  "bin": {
+    "bmad": "./tools/installer/bin/bmad.js",
+    "bmad-method": "./tools/installer/bin/bmad.js"
+  },
   "scripts": {
     "build": "node tools/cli.js build",
     "install": "node tools/installer/bin/bmad.js install",