docs(skills): enhance use-case-deployment and terraform-deploy skills

[iht]

Summary of Changes

This PR enriches the workspace Agent skills with operational best practices, complete workflows, and guardrails derived from live testing on Google Cloud:

1. use-case-deployment Skill Enhancements:

   • Pre-Flight Checks: Added service account collision checks, Shared VPC subnetwork verification, and Java toolchain LTS checks.
   • Complete Workflow Coverage: Added explicit CLI launch and validation commands for all 8 solution guides (GenAI/ML, ETL/Spanner CDC, CDP, Anomaly Detection, Marketing Intelligence, Clickstream Analytics, IoT Analytics, Log Replication).
   • Accurate Targets: Updated CDC output validation to target replica.events_changelog.
   • Safe Teardown Protocol: Documented the mandatory order of operations (cancelling active Dataflow streaming jobs before running terraform destroy).

2. terraform-deploy Skill Enhancements:

   • Updated terraform.tfvars templates to reflect the new lightweight variable schema (project_id, region, subnetwork, service_account_name, bucket_name, create_bucket, destroy_all_resources).
   • Integrated the pre-flight SA collision check and safe job teardown sequence into the deployment workflow.

[iht]

PR Review Summary

• Status: All 12 CI checks passed successfully.
• Verification:
  • Validated YAML frontmatter and markdown documentation formatting in .agents/skills/terraform-deploy/SKILL.md and .agents/skills/use-case-deployment/SKILL.md.
  • Verified pre-flight collision checks and Shared VPC guidelines.
  • Verified step-by-step launch and teardown commands for all 8 Dataflow reference pipelines.
• Conclusion: Ready for merge once external reviewer approval is granted.