Migrate ECS host default to AL2023

[0lut]

Summary

• switch the ECS EC2 host default from Amazon Linux 2 to a pinned Amazon Linux 2023 ARM64 ECS-optimized AMI
• pin the host AMI to the latest us-west-2 recommendation verified on June 8, 2026: ami-0ac01d3c8b7a34f9d / al2023-ami-ecs-hvm-2023.0.20260527-kernel-6.1-arm64
• add a guarded downtime migration script that scales the service to zero, terminates the existing host, and redeploys through the normal deploy/smoke path
• document the AL2023 maintenance-window flow, rollback override, post-migration Git remote checks, and how to intentionally advance the pinned AMI later

Deployment validation

• rebased on latest main f286fe3
• preview deployed with scripts/aws/deploy-preview.sh HEAD
  • VERSION_ID=0d2d298e772b
  • PUBLIC_BASE_URL=http://gitmirrorcache-arm-preview-alb-758126682.us-west-2.elb.amazonaws.com/v/0d2d298e772b
  • EC2 host i-0ec04cb3f080f7b15 is running ami-0ac01d3c8b7a34f9d
  • ECS service gmc-p-0d2d298e772b-ec2-api is ACTIVE desired=1 running=1
  • ALB target healthy on port 8080
• dev migrated with CONFIRM_DOWNTIME=true scripts/aws/migrate-ecs-host-al2023.sh
  • EC2 host i-01631e0cfaf39042d is running ami-0ac01d3c8b7a34f9d
  • ECS service gitmirrorcache-arm-ec2-api is ACTIVE desired=1 running=1
  • ALB target healthy on port 8080
• removed old AL2 preview stacks dc77d9c and cc125cc to clear affected preview hosts

Smoke tests

• bash -n scripts/aws/migrate-ecs-host-al2023.sh scripts/aws/deploy-ecs-ec2-ebs.sh scripts/aws/deploy-preview.sh scripts/aws/deploy-and-smoke.sh scripts/aws/smoke-test.sh
• git diff --check origin/main..HEAD
• preview /healthz and /metrics passed
• preview git ls-remote, shallow clone, and /v1/materialize against github.com/octocat/Hello-World passed
• dev /healthz and /metrics passed
• dev git ls-remote, shallow clone, and /v1/materialize against github.com/octocat/Hello-World passed
• active gitmirrorcache EC2 inventory checked: dev-arm and current preview ECS hosts are on ami-0ac01d3c8b7a34f9d; gitmirrorcache-dev-devbox is AL2023 x86_64

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.