`ToolOutput` pairs the model-facing text with the images a tool produced.
`ToolRegistry.execute` keeps its text-only contract; `execute_output` returns
the pair and the loop stores the images on the tool message, so a screenshot
tool can answer in pixels instead of a hex placeholder.
Only the text is capped by the tool-result limit; the char estimator now counts
a present image at its 1024-token ceiling, because the shrinkers never drop one.
