feat: reduce cluster to single zonal node with e2-small

Regional cluster (location=us-central1) creates 1 node per zone =
3 nodes total. Switch to zonal (us-central1-a) for a single node.
Also downsize machine_type from e2-medium to e2-small (2vCPU/2GB).

Add var.zone to variables.tf and GCP_ZONE to CI variables.
Update get-credentials to use --zone instead of --region.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>