Moves PLAN.md, STRIPE_PLAN.md, VITRINE_PLAN.md, WHATS_NEW_PLAN.md into a
gitignored plans/ directory instead of tracking them individually. Drops
SECURITY_AUDIT.md and its follow-up — session-scoped audit notes, already
captured in CHANGELOG.md and the actual code fixes.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>