chore(manifest): harden e2e workflow

[ScriptedAlchemy]

Summary

• replace the manifest e2e GitHub step with a script that watches the serve process so background exits bubble up
• add manifest:e2e scripts to package.json for reuse locally and in CI
• ensure serve cleanup/kill-port runs deterministically even when Nx dies

Testing

• node tools/scripts/run-manifest-e2e.mjs --mode=invalid

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Latest commit: 5a81541

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