fix: shell autocomplete bypasses preprocessing and completes attribute values

Bypass Execute() preprocessing for __complete/__completeNoDesc so Cobra's
built-in completion handles shell TAB without creating tasks. Add root
ValidArgsFunction for flexible syntax (e.g. "opal 1 de<TAB>" → delete),
attribute value completions (status:pending, priority:H, date synonyms),
and NoSpace directive on key: completions to avoid trailing space.

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

opal-task/cmd/root.go

@@ -54,8 +54,9 @@ var commandsWithModifiers = map[string]bool{
 var rootCmd = &cobra.Command{
 	Use:   "opal [filter] [command|report] [modifiers]",
 	Short: "Opal task manager - taskwarrior-inspired CLI task management",
-	Long: `Opal is a powerful command-line task manager. 
+	Long: `Opal is a powerful command-line task manager.
 It supports filtering, tags, priorities, projects, and recurring tasks.`,
+	Args: cobra.ArbitraryArgs,
 	Run: func(cmd *cobra.Command, args []string) {
 		// Default behavior: run configured default report (defaults to "list")
 		parsed := getParsedArgs(cmd)
@@ -87,6 +88,11 @@ func Execute() error {
 		if firstArg == "-h" || firstArg == "--help" || firstArg == "help" {
 			return rootCmd.Execute()
 		}
+		// Let Cobra's built-in completion machinery handle shell completions
+		// directly, bypassing preprocessing that would create tasks.
+		if firstArg == "__complete" || firstArg == "__completeNoDesc" {
+			return rootCmd.Execute()
+		}
 	}
 
 	// Preprocess arguments (read-only scan — os.Args is never mutated)