docs: add handover document and lessons learned for terraphim-ai migration

Comprehensive handover covering all 5 migration phases, merge plan for
PR #551, e2e verification results (49/49 checks pass), and technical
lessons learned (magic_pair precision, daachorse dedup, overlap detection,
symbolic embeddings, adjacency index optimization).

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

Author: AlexMikhalev