PR Summary

Low Risk
Low risk: changes are limited to CLI quiet-mode formatting and add optional stderr output for final prompt metadata without affecting stdout piping behavior.

Overview
Quiet output mode now still writes assistant text only to stdout, but additionally emits final ACP usage token counts and cost metadata to stderr when present in the session/prompt result.

This threads a stderr writer through createOutputFormatter, adds robust parsing/formatting for multiple usage/cost field shapes, and extends tests to assert the new stderr metadata lines.

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