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/root.go

@@ -32,6 +32,7 @@ var commandNames = []string{
 	"add", "done", "modify", "delete",
 	"start", "stop", "count", "projects", "tags",
 	"info", "edit", "server", "sync", "reports", "setup",
+	"version",
 }
 
 // Report names (dynamically populated)
@@ -47,9 +48,9 @@ var commandsWithModifiers = map[string]bool{
 }
 
 var rootCmd = &cobra.Command{
-	Use:   "opal",
+	Use:   "opal [filter] [command|report] [modifiers]",
 	Short: "Opal task manager - taskwarrior-inspired CLI task management",
-	Long: `Opal is a powerful command-line task manager inspired by taskwarrior.
+	Long: `Opal is a powerful command-line task manager. 
 It supports filtering, tags, priorities, projects, and recurring tasks.`,
 	Run: func(cmd *cobra.Command, args []string) {
 		// Default behavior: run configured default report (defaults to "list")
@@ -218,6 +219,10 @@ func init() {
 	rootCmd.AddCommand(infoCmd)
 	rootCmd.AddCommand(editCmd)
 	rootCmd.AddCommand(reportsCmd)
+	rootCmd.AddCommand(versionCmd)
+
+	// Enable --version flag on root command
+	rootCmd.Version = Version
 
 	// Add report commands dynamically
 	reportCommands := CreateReportCommands()