[Task](greenhouse): Enhance demo flow with secrets, Slack alerts, and Argo integration

[ArtieReus]

Task Description

Enhance demo flow with secrets, Slack alerts, and Argo integration

Sub-tasks

• Read demo token from the secret in the demo namespace.
  - [ ] Write a github action to write the link to argo into the PR.
• Notify in slack.
• Research if possible to notify argo to deploy (github action/hook).
• Improve security when polling a branch from within the initContainer
• Clean up greenhouse and related apps from github preview chore(ci): clean up greenhouse and related apps from github preview #1432