docs: add pipeline design analysis and rework plan

Add docs/ALGORITHM.md (design analysis: north-star UX principle, the
decision algorithm, tensions, and the decided design questions),
docs/WORKPLAN.md (phased, tickable implementation tracker), and
docs/questions.md (for recording blockers during implementation).

Captures a ground-up review of the dependabot-PR processing pipeline
and three rounds of independent design review.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: petemoore