Publish @flagsmith/backstage-plugin to NPM

[talissoncosta]

Summary

Publish the Flagsmith Backstage plugin to NPM for easy installation.

Parent Epic: #5641
Depends on: #6420 (Proxy migration), #6421 (/alpha export)

Tasks

• Update package.json name to @flagsmith/backstage-plugin
• Set up NPM publishing credentials
• Create GitHub Actions workflow for releases
• Publish initial version (0.1.0)
• Submit to Backstage plugin directory

Package.json Updates

{
  "name": "@flagsmith/backstage-plugin",
  "version": "0.1.0",
  "publishConfig": {
    "access": "public"
  }
}