Independent client/engine versioning with compatibility check

Split release.sh into release-client.sh and release-engine.sh for
independent release cadences. Client checks engine version on first
API call and hard-fails if engine is below MinEngineVersion.

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

client/cmd/root.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 
+	"github.com/kb-search/kb/internal/api"
 	"github.com/kb-search/kb/internal/config"
 	"github.com/spf13/cobra"
 )
@@ -11,6 +12,9 @@ import (
 // Version is set at build time via -ldflags.
 var Version = "dev"
 
+// MinEngineVersion is set at build time via -ldflags.
+var MinEngineVersion = "dev"
+
 var (
 	flagEngine string
 	flagFormat string
@@ -31,6 +35,7 @@ var rootCmd = &cobra.Command{
 }
 
 func init() {
+	api.SetVersionInfo(Version, MinEngineVersion)
 	rootCmd.Version = Version
 	rootCmd.PersistentFlags().StringVar(&flagEngine, "engine", "", "engine API URL")
 	rootCmd.PersistentFlags().StringVar(&flagFormat, "format", "", "output format (human|json)")