Add HeyGen plugin

[jrusso1020]

Summary

• restore the HeyGen plugin under plugins/heygen
• add the plugin to .agents/plugins/marketplace.json so it appears in the public catalog
• wire the plugin through .app.json and update the bundled skill docs to match the app-backed flow

Notes

• this is a fresh branch from origin/main, not a continuation of the prior reverted branch

[cching-openai]

On mobile, @jrusso1020 can you add a openai.yaml in your skills folder? See examples in the other plugins.