ci(os-49): release runner cutover

[jtoelke2]

Summary

Draft OS-131 / Phase 7 release runner cutover for the non-VM release path.

Related Issue

• OS-131
• Parent: OS-49

Changes

• Move release-canary.yml, release-dev.yml, and release-tag.yml CPU jobs from build-amd64 / build-arm64 to linux-amd64-cpu8 / linux-arm64-cpu8.
• Move reusable release workers driver-vm-linux.yml and deb-package.yml to the same shared CPU labels so release-dev/tag do not retain a hidden ARC dependency.
• Leave release-vm-dev.yml and release-vm-kernel.yml unchanged pending the OS-131 VM runtime decision.
• Keep trigger-wheel-publish on [self-hosted, nv] because it is the external GitLab publish bridge.
• Keep publish-fern-docs on ubuntu-latest.
• Update architecture docs with the Phase 7 split and remaining VM decision.

Merge Gate

• Draft only.
• Do not merge until Phase 6 cleanup PR ci(os-49): remove obsolete shadow PR workflows #1161 merges and one normal PR run confirms the obsolete shadow workflows no longer trigger.
• VM development release workflows remain a separate OS-131 decision path.

Testing

• git diff --check
• rg -n "build-amd64|build-arm64" .github/workflows/release-canary.yml .github/workflows/release-dev.yml .github/workflows/release-tag.yml .github/workflows/driver-vm-linux.yml .github/workflows/deb-package.yml returns no matches.
• 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 passed.

Checklist

• Follows Conventional Commits
• Scope limited to Phase 7 draft release runner cutover
• Architecture docs updated
• VM development release workflows left unchanged pending OS-131 decision

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