feat: replace branch preview app-of-apps with ArgoCD ApplicationSet PR generator

[Amertz08]

Summary

• Replaces the manual app-of-apps pattern (GitHub Actions writing Application YAMLs to deploy/argocd/branches/) with an ArgoCD ApplicationSet using the Pull Request generator
• ArgoCD now polls GitHub directly for open PRs and manages preview Application lifecycle automatically — no more commit noise on main for branch push/delete events
• Removes reusable-argocd-register.yml and reusable-argocd-cleanup.yml workflows entirely; removes register-branch and cleanup-branch jobs from the API workflow

Behavioral Changes

Aspect	Before	After
Trigger	Branch push	PR open
Namespace	gitops-example-<safe-branch>	gitops-example-pr-<number>
Cleanup	Workflow on delete event	Automatic on PR close/merge
Branches without PRs	Get a preview	No preview
Commit noise on main	Yes (one per branch push/delete)	None
Polling latency	Immediate	Up to 60 s

Cluster Prerequisites

Before merging, create the GitHub token secret in the cluster:

kubectl -n argocd create secret generic github-token \
  --from-literal=token=<YOUR_TOKEN>

After merging, delete the now-inert umbrella Application:

kubectl -n argocd delete application gitops-example-branches

Test plan

• Create github-token secret in the argocd namespace
• Merge this PR
• Open a test PR on a branch that has had cmd/api/main.go changed (so a branch image exists)
• Within ~60 s confirm kubectl -n argocd get applications | grep pr- shows gitops-example-pr-<N>
• Confirm pod is Running in gitops-example-pr-<N> namespace
• Close/merge the test PR and confirm Application and namespace are removed within ~60 s
• Push to a branch with no open PR — confirm no Application is created

🤖 Generated with Claude Code