revert: delete stale phase-1 playable-slice plan

Phasing approach changed from vertical-slice to engine-first per the
description in superpowers analysis: P1 ships engine core with vitest
verification only (no UI), P2 layers AI personalities, P3 builds UI
screens, P4 polishes. Replacement plan lands in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: emp3thy