Self-hosted runners lack `gh` (+ Node 20 deprecation): verification-gate PR comment can't post on smithy

[avrabe]

Surfaced by the v1.55 smithy runner migration (#149).

Two infra gaps the migration exposed

1. gh CLI absent on the smithy Hetzner runners. tools/post_verification_comment.py shells out to gh repo view / gh pr comment; on GitHub-hosted ubuntu-latest gh is preinstalled, on pulseengine-ci-01-* it is not → FileNotFoundError: 'gh'. Mitigated in feat(ci): v1.55 — CI sovereignty: smithy self-hosted runners + Kani roll-up gate #149 (the comment step is now continue-on-error + the script soft-fails), so the gate verdict is correct — BUT the sticky verification comment no longer posts on self-hosted PRs. To restore it: either add gh to the smithy runner image (smithy Ansible), or switch the script to the GitHub REST API via GITHUB_TOKEN + curl/urllib (no gh dependency — preferred, keeps the workflow runner-agnostic).

2. Node 20 action deprecation. actions/upload-artifact@v4 (and others) warn: Node 20 is forced to Node 24 on 2026-06-16. Bump pinned action versions before then across relay's workflows.

Neither blocks a merge; both are CI hygiene for the now-self-hosted pipeline. Candidate for a v1.55.x or v1.58 (hygiene) follow-up.

🤖 Generated with Claude Code

[avrabe]

Disposition: Node-20 half DONE; gh-on-self-hosted half scoped (low priority).

• Node-20 deprecation — done (ci: Node-24 opt-in for artifact actions + coverage checkout v5 (#153) #159): the three artifact-using workflows opt into Node 24 (FORCE_JAVASCRIPT_ACTIONS_TO_NODE24), and coverage.yml's lone checkout@v4 → @v5. Clean past the 2026-06-16 forced cutover.
• gh absent on smithy — the verification-gate sticky comment can't post on self-hosted runners. Already mitigated (the step is continue-on-error + the script soft-fails, since feat(ci): v1.55 — CI sovereignty: smithy self-hosted runners + Kani roll-up gate #149), so it's non-fatal — the gate verdict is unaffected. The clean fix to restore the comment is switching tools/post_verification_comment.py from the gh CLI to the GitHub REST API via GITHUB_TOKEN + urllib (runner-agnostic). Low priority given it's already non-fatal. Keeping open for that REST-API switch.