Support custom domain mapping in gapp.yaml

[krisrowe]

Summary

Allow solutions to declare a custom domain in gapp.yaml. gapp creates the Cloud Run domain mapping via terraform. DNS configuration remains manual (gapp can't manage external DNS providers).

Proposed format

domain: myapp.example.com

Behavior

• Terraform creates google_cloud_run_domain_mapping resource when domain is set
• Cloud Run provisions SSL certificate automatically
• Both the default Cloud Run URL and the custom domain work simultaneously
• gapp status reports both URLs and DNS/SSL readiness state
• gapp mcp connect includes the custom domain in connection instructions when active
• Deploy skill guides user through the manual DNS CNAME step after first deploy

What gapp does NOT do

• Manage DNS records (external provider — manual CNAME to ghs.googlehosted.com)
• Block deployment if DNS isn't configured yet — the default URL always works

Implementation

• Add domain variable to terraform module
• Add google_cloud_run_domain_mapping resource (conditional on domain being set)
• Update _build_tfvars to pass domain
• Update status.py to check domain mapping state via gcloud
• Update deploy skill with DNS guidance