Fix npx execution issue with bmad CLI

- Added wrapper script (bmad.js) at root to handle npx execution context
- Fixed module resolution in tools/installer/bin/bmad.js for both local and npx contexts
- Updated package.json bin paths to use the wrapper script
- Handles temporary npx directories properly using execSync

This fixes the issue where `npx github:bmadcode/BMAD-METHOD#v4-alpha bmad`
was dropping into a shell instead of executing the command.

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

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

package.json

@@ -4,8 +4,8 @@
   "description": "Breakthrough Method of Agile AI-driven Development",
   "main": "tools/cli.js",
   "bin": {
-    "bmad": "./tools/installer/bin/bmad.js",
-    "bmad-method": "./tools/installer/bin/bmad.js"
+    "bmad": "./bmad.js",
+    "bmad-method": "./bmad.js"
   },
   "scripts": {
     "build": "node tools/cli.js build",