improvement: implement ArgoCD Hydrator for manifest pre-rendering

[ixxeL2097]

Context

ArgoCD's Manifest Hydrator (introduced as an experimental feature in ArgoCD 2.13+) allows pre-rendering Helm/Kustomize manifests into plain YAML and committing them back to Git, enabling fully GitOps-auditable manifests and faster sync (no server-side rendering).

Goal

Evaluate and implement the ArgoCD Hydrator in the genmachine cluster to:

• Pre-render all ApplicationSet-managed Helm charts into hydrated manifests stored in Git
• Make the rendered YAML auditable and diffable in PRs (Renovate diff comments become exact k8s manifests, not values changes)
• Reduce ArgoCD control-plane load (no Helm rendering at sync time)
• Enable argocd app diff without needing cluster access

Tasks

• Enable the Hydrator feature gate in the ArgoCD Helm values (gitops/bootstrap/genmachine/)
• Configure a HydrationController or equivalent manifest hydration pipeline
• Define the target branch/path where hydrated manifests are committed (e.g. hydrated/ or a dedicated branch)
• Update ApplicationSets to source from hydrated manifests path
• Validate that Renovate PRs show the actual rendered diff (not just values changes)
• Document the hydration flow in docs/

References

• https://argo-cd.readthedocs.io/en/latest/operator-manual/manifest-hydration/
• https://github.com/argoproj/argo-cd/blob/master/docs/operator-manual/manifest-hydration.md