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/modify.go

@@ -85,6 +85,12 @@ func modifyTasks(filterArgs, modifierArgs []string) error {
 		return fmt.Errorf("no tasks matched filter")
 	}
 
+	if dryRunFlag {
+		fmt.Print(engine.FormatTaskConfirmList("modify", tasks, ws))
+		fmt.Println("Dry run — no changes made.")
+		return nil
+	}
+
 	// Confirm if multiple tasks or no filters specified
 	if len(tasks) > 1 || len(filterArgs) == 0 {
 		fmt.Print(engine.FormatTaskConfirmList("modify", tasks, ws))