For embedding companions (e.g. trollshell WebKitGTK) that supply their
own header/input chrome around the per-agent page and only want the
raw terminal feed. Comma-separated so both axes can hide in one param:
?hide=header,input. Zeroes --agent-header-h/--agent-composer-h via body
classes rather than conditionally computing padding, so every dependent
calc() (overlay offset, terminal scroll padding) collapses in one place.
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