Slack link unfurls (inline message previews) are enabled by default
when unfurl_links is not explicitly set in chat.postMessage. This means
bot messages containing Slack message links or URLs automatically expand
into rich preview cards, which can be noisy in channels.
Default unfurl_links to false so outbound messages don't show inline
link previews unless the operator explicitly opts in via:
channels.slack.unfurlLinks: true
unfurlMedia remains opt-in (only sent when explicitly configured).
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