Remove legacy safrat-assistant/ reference implementation
The original SQLite-backed assistant (flat src/ layout, formerly safrat-assistant_v3/) was kept for rollback during the ArchiPy migration. The port in assistant/ is verified and is now the single source of truth, so the legacy directory is removed (23 files, including its pip requirements.txt — the repo is uv-only now).
Docs updated to match:
- CLAUDE.md / README.md: drop the two-implementations framing and the "keep the two in sync" rule; note assistant/ is the only implementation.
- assistant/ARCHITECTURE.md: reword the Context-key compatibility note to reference Postgres session restore instead of the removed reference impl.
- MIGRATION.md: mark the legacy dir as removed (historical narrative kept).
- DEPLOY.md: add a "superseded, see README" banner and fix the stale build-path / .env directory references (./assistant, assistant/.env).
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com