🔄 refactor(actions): migrate from Elastic Beanstalk to ECS deployment

[Rhodri-Morgan]

Summary

Migrate deployment system from Elastic Beanstalk to ECS. Removes the EB deployment action and adds a new deploy-ecs action that uses AWS ECS task definitions with automatic rollback on failure. Standardizes all action file extensions from .yml to .yaml. Adds comprehensive AGENTS.md and CLAUDE.md documentation.

Changes

• Removed: deploy_elastic_beanstalk/action.yml — Replaced with ECS-based deployment
• Added: deploy-ecs/action.yaml — New action for ECS service deployments with task definition management and automatic rollback
• Updated: README.md — Removed EB deployment documentation and examples, added ECS deployment section with simplified example workflow
• Renamed: build/action.yml → build/action.yaml, push/action.yml → push/action.yaml — Standardized to .yaml extension
• Added: AGENTS.md — Comprehensive documentation covering:
  • Architecture patterns (SSM parameter tracking, task definition cloning, rollback, OIDC)
  • AWS OIDC authentication details and GitHub secrets
  • Scripts directory purpose (shared scripts fetched at runtime)
  • Development guidelines for actions
• Added: CLAUDE.md — Entry point that references AGENTS.md

Key Features of deploy-ecs

• Automatically strips 'v' prefix from image tags (e.g., v2026.03.26 → 2026.03.26)
• Reads current task definition, updates image tag, registers new revision
• Polls for deployment completion with configurable timeout
• Automatically rolls back to previous task definition on failure
• Updates SSM parameter only after successful deployment
• Infrastructure changes (env vars, resources) managed via Terraform, not action

Testing

• Verified all changes via git diff against origin/master
• Action files formatted correctly with composite action structure
• Documentation links verified (CLAUDE.md → AGENTS.md)
• README.md examples updated with ECS-specific configuration

Checklist

• Code follows style guidelines
• Actions use composite action structure with bash steps
• Documentation complete and accurate
• AWS infrastructure management correctly delegated to Terraform
• Old Elastic Beanstalk references completely removed