Add a k3s deploy path for the dotnet-templates-fullstack app

[ChrisSimmons]

Context

Part of the Azure exodus epic NetChris/dotnet-templates-fullstack#30. Today, api-push.yml/api-release.yml/app-push.yml/app-release.yml in dotnet-templates-fullstack call this repo's deploy-azure-container-app.yml reusable workflow after build-test-publish-dotnet-app-component.yml publishes an image.

The shape of this issue depends entirely on the outcome of NetChris/Home-infrastructure#283 (deploy mechanism investigation):

• If a self-hosted runner is chosen: add a new reusable workflow (e.g. deploy-k3s.yml) that kubectl/kustomize applies the image tag to the cluster, mirroring deploy-azure-container-app.yml's shape.
• If GitOps (Flux/ArgoCD) is chosen: this may reduce to removing the deploy job entirely - publishing the image (or updating a manifest repo/tag) is enough, and the in-cluster controller handles the rest. In that case this issue's scope shrinks to documenting the new "done" condition for the push/release workflows.
• If deploy stays manual: this issue is not needed - close as not planned once that's confirmed.

Done when

• Deploy mechanism decided (NetChris/Home-infrastructure#283)
• Reusable workflow added or explicitly deemed unnecessary, with the reasoning recorded here