refactor: extract pi-kit to standalone repository

[ElbertePlinio]

Final slice of #54: remove pi-kit from the monorepo now that the standalone repo (ElbertePlinio/pi-kit, CI green) owns it and all machine consumers are cut over (dotfiles PR #16, applied and behaviorally verified).

What changed

• git rm -r packages/pi-kit; bun.lock regenerated without the workspace member
• pi-kit conventions removed from AGENTS.md (moved to the standalone repo)
• Three pi-kit design/plan docs archived under docs/superpowers/archive/ with extraction notes
• Coverage thresholds rebaselined to the remaining packages' actual level (statements 73→72, branches 66→64, functions 84→83, lines 72) — maintainer-approved; no remaining-package coverage changed, pi-kit's 184 tests enforce their own thresholds in its repo

Tested

• bun run check green at root: typecheck, 260 tests passed (25 skipped), coverage above rebaselined thresholds, build success
• Repo-wide sweep: zero active pi-kit references outside the archive

Review

Main-agent review + required CI (mechanical removal; the one judgment call — thresholds — was explicitly maintainer-approved).

Closes #54

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