ci: cut over non-release workflows to shared runners

[jtoelke2]

Summary

Cut over OS-130 / OS-49 Phase 6 non-release CI workflows from ARC build-* runners to the supported shared runner path.

Related Issue

Related to OS-130 / OS-49.

Changes

• Move Branch Checks onto copied-PR pull-request/* triggers with pr-gate and linux-{amd64,arm64}-cpu8.
• Convert CI image publishing to native per-arch shared-runner builds and merge the multi-arch manifest after both arch images push.
• Move Branch E2E / reusable E2E defaults to shared CPU runners and remove old build-* runner plumbing from those call paths.
• Update CI docs for the copied-PR branch checks flow.

Testing

• git diff --check
• mise run markdown:lint
• env PATH=/home/jtoelke/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin /home/jtoelke/.local/bin/mise run pre-commit

Notes

• Draft PR for review only. Do not merge until the replacement check names and branch protection cutover are agreed.
• CI image publishing still only runs on main push or manual dispatch.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Documentation updated

[copy-pr-bot]

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