Add missing plugins to marketplace.json and create automation script

- Add context-engineering, gem-team, and ospo-sponsorship to marketplace.json
- Create eng/generate-marketplace.mjs to automatically generate marketplace.json
- Update package.json build script to include marketplace generation
- Update GitHub workflow to trigger on plugin changes

Co-authored-by: aaronpowell <434140+aaronpowell@users.noreply.github.com>

package.json

@@ -6,7 +6,7 @@
   "private": true,
   "scripts": {
     "start": "npm run build",
-    "build": "node ./eng/update-readme.mjs",
+    "build": "node ./eng/update-readme.mjs && node ./eng/generate-marketplace.mjs",
     "contributors:add": "all-contributors add",
     "contributors:report": "node ./eng/contributor-report.mjs",
     "contributors:generate": "all-contributors generate",
@@ -17,6 +17,7 @@
     "skill:create": "node ./eng/create-skill.mjs",
     "plugin:migrate": "node ./eng/collection-to-plugin.mjs",
     "plugin:refresh": "PLUGIN_MODE=refresh node ./eng/collection-to-plugin.mjs",
+    "plugin:generate-marketplace": "node ./eng/generate-marketplace.mjs",
     "website:data": "node ./eng/generate-website-data.mjs",
     "website:dev": "npm run website:data && npm run --prefix website dev",
     "website:build": "npm run build && npm run website:data && npm run --prefix website build",