Child calls _redirect_output after env setup: each configured log path is
opened append-mode and dup2'd onto fd 1/2. None/inherit keeps the inherited
fd; stderr stdout shares stdout's fd. Rotation stays external.
Add tests for inherit, append flags, file dup2, and stderr-to-stdout.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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