ci: move CI to GitHub-hosted runners

[kimo-ice]

Why

We're decommissioning the self-hosted ARC runner cluster (arc-runner-set* labels). It costs ~$1,700/month, and CI uses about 0.3% of its CPU. Standard GitHub-hosted runners are free for public repos and give every job a fresh VM. This PR moves every job in this repo off ARC.

What changes

• arc-runner-set becomes ubuntu-latest, and arc-runner-set-arm64 becomes ubuntu-24.04-arm.
• Files: ci.yml, docker-build.yml
• All ci.yml jobs passed on hosted runners in chore(repo): benchmark CI on GitHub-hosted runners (do not merge) #5. test took ~12 min (ARC p50 21.0).
• Arm64 image builds move to native ubuntu-24.04-arm runners.

No job depends on the ARC runner for credentials or network access. GCP access is authenticated explicitly in the workflows (WIF or a key secret), and the runner cluster has no fixed egress IP.

Test plan

• PR-triggered jobs pass on this PR.
• After merge: the first push/release-triggered runs (image builds, deploys, release-please) succeed. Watch durations for 3 days against the ARC baseline; a regression is fixed forward or the workflow reverted.