feat(executor): store TaskfileGraph for lazy resolution

Store the TaskfileGraph in the Executor so it can be used for lazy
variable resolution when SCOPED_INCLUDES experiment is enabled.

The graph is now preserved after reading, before merging into the
final Taskfile. This allows traversing the DAG at runtime to resolve
variables from the correct scope.

executor.go

@@ -63,6 +63,7 @@ type (
 
 		// Internal
 		Taskfile           *ast.Taskfile
+		Graph              *ast.TaskfileGraph
 		Logger             *logger.Logger
 		Compiler           *Compiler
 		Output             output.Output

setup.go

@@ -104,6 +104,7 @@ func (e *Executor) readTaskfile(node taskfile.Node) error {
 		}
 		return err
 	}
+	e.Graph = graph
 	if e.Taskfile, err = graph.Merge(); err != nil {
 		return err
 	}