docs: retire superpowers/ layout — keep design spec under docs/specs/
The superpowers workflow methodology is no longer in use. Move the methodology-neutral design spec to docs/specs/ as the project's design ledger, drop the superpowers-bound implementation plan (it referenced superpowers sub-skills), and remove the now-dead /docs/superpowers/ .gitignore entry. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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