When DISCORD_IGNORE_NO_MENTION is true (default), the bot ignores
messages without @mention. However, this check ran before evaluating
free_response_channels, so messages in free-response channels were
wrongly dropped unless they contained a mention.
This change adds a carve-out: if the message lands in a channel that
is configured as a free response channel (or its parent category is),
the ignore-no-mention rule is skipped.
Also removes the unconditional skip_thread for free response channels
so that auto_thread still creates threads there unless explicitly
disabled via DISCORD_NO_THREAD_CHANNELS.
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