Bring the original Overview gating contract forward to the new
footnotes block: any non-empty `instance.overview` /
AGNES_INSTANCE_OVERVIEW value enables the footnotes, an empty
value hides them. The raw yaml HTML body is no longer rendered
(the static product framing replaces it) — operators relying on
custom Overview HTML should migrate that content to
instance.custom_scripts or admin-edited news.
Preserves the explanatory comments from the original Overview
section (operator-owned, opt-in, no dismiss button) so future
readers understand why the block is gated.
Splits the test into two halves matching the original gating
pattern: footnotes appear when the flag is set, hidden when
unset. Uses "Get the most out of it" as the marker (unique to
the footnotes copy) since "What leaves your machine" still
appears in the untouched session-privacy annotation lower on
the page.
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