Free-response channels skip auto-threading by default so the bot replies
inline (lightweight chat mode). This prevented users who wanted BOTH
mention-free replies AND per-conversation threads from getting either.
Add a new opt-in `discord.free_response_auto_thread` (env:
`DISCORD_FREE_RESPONSE_AUTO_THREAD`, default false) that, when true,
re-enables auto-threading in free-response channels. Voice-linked
channels continue to skip auto-thread regardless, and the flag is
gated behind the global `DISCORD_AUTO_THREAD=true`.
Default behavior is unchanged; all 291 existing discord tests pass.
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