Add GPU device control, Docker support, and v2 client-server design

- Add configurable device selection for embeddings (embedding.device) and
  Docling ingestion (ingestion.device) with env var overrides (KB_DEVICE,
  KB_INGEST_DEVICE) to control GPU/CPU usage per component
- Add `kb doctor` command for safe GPU diagnostics
- Add Dockerfile (NVIDIA CUDA) and compose.yaml for containerised GPU usage
- Add OpenSpec v2 change (kb-v2-client-server): proposal, design, specs, and
  tasks for client-server architecture with Go CLI, FastAPI engine, async
  ingestion queue, and GPU-vendor-agnostic Docker deployment
- Add uv.lock for reproducible installs
- Gitignore examples/ directory (test data only)

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

compose.yaml

@@ -0,0 +1,20 @@
+services:
+  kb:
+    build: .
+    runtime: nvidia
+    deploy:
+      resources:
+        reservations:
+          devices:
+            - driver: nvidia
+              count: 1
+              capabilities: [gpu]
+    volumes:
+      - kb-data:/data
+      - ./examples:/examples:ro
+    # Override entrypoint for interactive use:
+    #   docker compose run kb search "query"
+    #   docker compose run kb add /examples/car.pdf
+
+volumes:
+  kb-data: