Add GKE-backed DevSpace starter-pack target

[michaelw]

Summary

• add an ephemeral GKE Terraform target with Cloud DNS, Artifact Registry, Workload Identity, and IAP IAM wiring
• add context-derived DevSpace cluster selection, tool preflight, host DNS helpers, GKE Gateway/route profiles, and the starter-pack cluster environment ConfigMap
• add GKE smoke/install diagnostics plus docs for GKE setup, authz conventions, observability, and troubleshooting

See also: michaelw/ext-authz-token-exchange#8

Validation

• bash -n scripts/devspace-cluster-backend.sh
• bash -n scripts/check-tools.sh
• bash -n scripts/cluster-dns.sh
• yq e '.' devspace.yaml >/dev/null
• git diff --check
• go test ./tests/e2e/cmd/smoke
• go test -run '^$' ./tests/install
• scanned branch diff for live project/account identifiers; ignored local TODO/runtime files are not included

Notes

• This is a draft PR so CI can validate the full change set before review.
• Runtime state remains ignored: .devspace/, local/, Terraform state/tfvars/locks, and Helm lockfiles are intentionally not committed.