feat(runners): reference launch template $Latest version in ASG

[coderbirju]

What

Add a CFN escape hatch in lib/asg-runner-stack.ts so each runner ASG references its launch template by id at the $Latest version, instead of the version number pinned at synth time.

const cfnAsg = asg.node.defaultChild as autoscaling.CfnAutoScalingGroup;
cfnAsg.addPropertyOverride('LaunchTemplate.LaunchTemplateId', lt.launchTemplateId);
cfnAsg.addPropertyOverride('LaunchTemplate.Version', '$Latest');
cfnAsg.addPropertyDeletionOverride('LaunchTemplate.LaunchTemplateName');

Why

The L2 AutoScalingGroup pins Version to LatestVersionNumber captured at deploy time. By overriding to the literal $Latest, new instance launches, scale-outs, and instance refreshes always pick up the newest launch template version whenever a change is deployed (or a new LT version is created out-of-band), rather than a synth-time-pinned number. Applies to all runner ASGs (mac/windows/linux).

Tradeoff

$Latest affects new instance launches. Existing instances are replaced by the rolling UpdatePolicy on deploy. A deploy that changes only the launch template may not by itself force a rolling replacement (the ASG's Version string stays "$Latest", so CloudFormation sees no ASG diff) — those instances pick up the latest version when they next cycle/refresh.

Testing

• npm run build (tsc) passes.
• npm test (jest): 12 suites / 18 tests pass.
• Added a focused unit assertion in test/asg-runner-stack.test.ts verifying the ASG's LaunchTemplate has Version: '$Latest', a LaunchTemplateId, and no LaunchTemplateName. No existing assertions were weakened (the prior suite did not assert an ASG LaunchTemplate version).
• npx cdk synth of a linux ASG stack renders LaunchTemplate: { LaunchTemplateId: { Ref: ... }, Version: "$Latest" }.