Cleanup: decide archive vs keep for spool-rack repo

[pewpewpotato]

Problem

spool-rack is product-sunset but the repo artifact may still look live (README/About/CI).

Who it’s for

Anyone discovering spool-rack on GitHub; maintainers avoiding false “ship this” signals.

Acceptance criteria

• Explicit decision: archive vs keep-for-history only
• If archive: GitHub archive + README banner pointing at Spool git-SoT / migrate-once
• About/CI don’t imply an active Rack product path
• Spool docs don’t link to Rack as a happy-path dependency

Priority

Second of the post-pivot cleanup slice (after skills audit).

Owner

Werner (decision) + Spool Technical Lead (apply)