v2 restructure: Go client, Docker engine, release tooling

- Remove v1 Python CLI (src/kb_search/, tests/, root pyproject.toml, uv.lock, .venv)
- Add Go client with cross-platform build (client/)
- Add FastAPI engine with NVIDIA and multi-stage ROCm Dockerfiles (engine/)
- Add VERSION files for client and engine, wired into builds
- Add release.sh for automated build, tag, release, and Docker push
- Update README with build/release docs and ROCm migration note
- Clean up .gitignore for v2 project structure

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

engine/compose.nvidia.yaml

@@ -0,0 +1,24 @@
+services:
+  kb-engine:
+    build:
+      context: .
+      dockerfile: Dockerfile.nvidia
+    runtime: nvidia
+    deploy:
+      resources:
+        reservations:
+          devices:
+            - driver: nvidia
+              count: 1
+              capabilities: [gpu]
+    ports:
+      - "${KB_PORT:-8000}:8000"
+    volumes:
+      - ${KB_DATA_PATH:-./data}:/data
+    environment:
+      - KB_MODEL=${KB_MODEL:-all-MiniLM-L6-v2}
+      - KB_DEVICE=${KB_DEVICE:-auto}
+      - KB_INGEST_DEVICE=${KB_INGEST_DEVICE:-auto}
+      - KB_API_KEY=${KB_API_KEY:-}
+      - KB_SEARCH_THRESHOLD=${KB_SEARCH_THRESHOLD:-0.01}
+    restart: unless-stopped