chore: untrack internal design doc that was committed by accident

`pickle-redesign-plan.md` is an off-tree working doc for PR1/PR2
sequencing decisions, not user-facing documentation. It was added by
a stray `git add -A` in the previous commit. Untrack it; the file
stays in the working tree as untracked for local reference.

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

Author: timsaucer