Add CPU-only Docker image and fix release tag naming

- Add Dockerfile.cpu and compose.cpu.yaml for CPU-only deployments
- Use sentence-transformers[onnx] + CPU-only torch for ~4x smaller image
- Fix release script: separate git tags (engine-v*) from Docker tags (v*)
- Add CPU image to release build/push pipeline
- Update README with CPU deployment instructions

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

engine/compose.cpu.yaml

@@ -0,0 +1,17 @@
+services:
+  kb-engine:
+    build:
+      context: .
+      dockerfile: Dockerfile.cpu
+    ports:
+      - "${KB_PORT:-8000}:8000"
+    volumes:
+      - ${KB_DATA_PATH:-./data}:/data
+    environment:
+      - KB_MODEL=${KB_MODEL:-all-MiniLM-L6-v2}
+      - KB_DEVICE=cpu
+      - KB_INGEST_DEVICE=cpu
+      - KB_API_KEY=${KB_API_KEY:-}
+      - KB_SEARCH_THRESHOLD=${KB_SEARCH_THRESHOLD:-0.01}
+      - HF_HUB_OFFLINE=${HF_HUB_OFFLINE:-}
+    restart: unless-stopped