Free-response channels already bypassed the @mention gate so users could
chat inline with the bot, but auto-threading still fired on every
message — spinning off a thread per message and defeating the
lightweight-chat purpose.
Fix: fold `is_free_channel` into `skip_thread` so threading is skipped
whenever the channel is in DISCORD_FREE_RESPONSE_CHANNELS (via env or
discord.free_response_channels in config.yaml).
Net change: one line in _handle_message + one regression test.
Partially addresses #9399. Authored by @Hypn0sis (salvaged from PR #9650;
the bundled 'smart' auto-thread mode from that PR was dropped in favor
of deterministic true/false semantics).
93fe4b357d