feat: add --dry-run flag to action commands

Adds a persistent --dry-run flag that shows matched tasks without
performing mutations. Supported on done, delete, modify, start, and stop
commands. Also fixes preprocessArgs to skip flag-like args when
identifying commands, preventing flags from being treated as filters.

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

opal-task/cmd/done.go

@@ -66,6 +66,12 @@ func completeTasks(args []string) error {
 		return fmt.Errorf("no tasks matched filter")
 	}
 
+	if dryRunFlag {
+		fmt.Print(engine.FormatTaskConfirmList("complete", tasks, ws))
+		fmt.Println("Dry run — no changes made.")
+		return nil
+	}
+
 	if len(tasks) > 1 {
 		fmt.Print(engine.FormatTaskConfirmList("complete", tasks, ws))
 		fmt.Printf("Proceed? (y/N): ")