ci: require every mandatory workflow job to succeed

[binaryaaron]

What changed and why

Aggregate every mandatory job into CI Status and fail on failures, cancellation, missing results and unexpected skips. Document reruns and stacked-branch behavior.

Stack position: 7/12. Base: codex/usersim-draft-pr06-container. Merge in stack order.

How it was verified

CI aggregate behavior was covered by focused checker/workflow tests; this stage retains the reviewed incremental CI changes. Make-stage runner adaptations were checked at PR5/6.

The latest cleanup used focused checks and independent source review. Earlier full matrix/native/container results retain their original revision; those broad runs and all twelve Arc reviews were not repeated after the cleanup. GitHub CI results are pending.

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