docs(superpowers): add spec + plan for learn/remember quality-gate verification

Paired with the SKILL.md edits in 683ae5b / a089031 / 0f7ed7b. The spec
captures the three-gate redesign (sharpened Recurs, verification-driven
Right home with three outcome rows) and the non-goals (no Go-side
changes, no writes to external homes). The plan documents the
baseline→edit→GREEN pressure-test cycle executed to land the change.

AI-Used: [claude]

Author: toejough