feat: expose health_check_type for runner manager ASG

[lorenzstorm1]

Summary

• Adds health_check_type and health_check_grace_period to the runner_instance variable
• Wires them into the aws_autoscaling_group.gitlab_runner_instance resource
• Defaults unchanged: "EC2" health check type, 0 grace period

Motivation

The runner manager ASG uses EC2 health checks, which only detect hardware-level failures. OS-level freezes (OOM, kernel panic that doesn't trigger instance stop, EBS I/O stalls) leave the instance appearing healthy from the hypervisor's perspective while all userspace processes are dead.

With this change, users can attach a target group pointing at the gitlab-runner Prometheus endpoint (:9252/metrics) and set health_check_type = "ELB" to get automatic replacement of unresponsive manager instances.

Usage

runner_instance = {
  name              = "gitlab-runner"
  type              = "t4g.medium"
  health_check_type = "ELB"
  health_check_grace_period = 120
}

Combined with an ALB/NLB target group attached to the ASG.

Test plan

• Verify terraform plan shows no changes when using defaults ("EC2", 0)
• Verify setting health_check_type = "ELB" propagates to the ASG resource

🤖 Generated with Claude Code

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