feat: IMP-13 — add version command with build-time variables

opal version (and opal --version) prints version, commit, and build
date. Values are set via ldflags at build time; defaults to "dev" for
local builds. VERSION file added at 0.1.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/version.go

@@ -0,0 +1,26 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+// Set via ldflags at build time:
+//
+//	go build -ldflags "-X git.jnss.me/joakim/opal/cmd.Version=$(cat VERSION)
+//	  -X git.jnss.me/joakim/opal/cmd.Commit=$(git rev-parse --short HEAD)
+//	  -X git.jnss.me/joakim/opal/cmd.BuildDate=$(date -u +%Y-%m-%d)"
+var (
+	Version   = "dev"
+	Commit    = "unknown"
+	BuildDate = "unknown"
+)
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print version information",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Printf("opal %s (%s) built %s\n", Version, Commit, BuildDate)
+	},
+}