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>

client/cmd/root.go

@@ -0,0 +1,46 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/kb-search/kb/internal/config"
+	"github.com/spf13/cobra"
+)
+
+// Version is set at build time via -ldflags.
+var Version = "dev"
+
+var (
+	flagEngine string
+	flagFormat string
+	flagAPIKey string
+)
+
+var rootCmd = &cobra.Command{
+	Use:   "kb",
+	Short: "kb-search CLI client",
+	Long:  "A CLI client for the kb-search v2 engine API.",
+	PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
+		if err := config.Load(); err != nil {
+			return err
+		}
+		config.ApplyFlags(flagEngine, flagFormat, flagAPIKey)
+		return nil
+	},
+}
+
+func init() {
+	rootCmd.Version = Version
+	rootCmd.PersistentFlags().StringVar(&flagEngine, "engine", "", "engine API URL")
+	rootCmd.PersistentFlags().StringVar(&flagFormat, "format", "", "output format (human|json)")
+	rootCmd.PersistentFlags().StringVar(&flagAPIKey, "api-key", "", "API key for authentication")
+}
+
+// Execute runs the root command.
+func Execute() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}