docs(requirements): add complete contextual user model requirements (phases 0-6)

Full rewrite covering all 8 phases with consistent per-phase template.
Fixes strategy pattern contradiction from original Phase 1 requirements,
documents completed phases 0A-4 retrospectively, adds prescriptive
requirements for TODO phases 5-6 (legacy removal and migration).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tianjianjiang