chore: rename runner tag to ephemeral-ci-runner

[jack-champagne]

Summary

• Renames the EC2 instance tag from gpu-runner=true to ephemeral-ci-runner=true in both CI and benchmark workflows
• Coordinated with harmoniqs/aws-infra#56 which generalizes the IAM termination policy to support non-GPU ephemeral runners (devtools nightly rebase bot)

Changes

File	Change
.github/workflows/CI.yml	gpu-runner -> ephemeral-ci-runner in aws-resource-tags
.github/workflows/gpu-benchmark.yml	gpu-runner -> ephemeral-ci-runner in aws-resource-tags

Deployment order

1. Merge this PR first — CuQuantum.jl workflows start tagging with the new name
2. Then deploy harmoniqs/aws-infra#56 — IAM policy switches to requiring the new tag for termination

This order is safe because:

• Before aws-infra deploys: the IAM policy still checks for gpu-runner=true, but CuQuantum workflows now tag with ephemeral-ci-runner=true. The stop-runner job will fail to terminate. Risk window is minimal if both are merged close together.
• Alternatively, deploy aws-infra first: old CuQuantum workflows tag gpu-runner=true but IAM expects ephemeral-ci-runner=true. Same risk.
• Safest: merge both, then deploy aws-infra immediately after.

Test plan

• Verify CI workflow still runs GPU tests on PR (tag change doesn't affect instance launch, only termination permissions)
• After aws-infra#56 is deployed, verify stop-runner job successfully terminates the instance

🤖 Generated with Claude Code