fix(discord): free-response channels skip auto-threading

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).

tests/gateway/test_discord_free_response.py

@@ -385,6 +385,33 @@ async def test_discord_voice_linked_channel_skips_mention_requirement_and_auto_t
     assert event.source.chat_type == "group"
 
 
+@pytest.mark.asyncio
+async def test_discord_free_channel_skips_auto_thread(adapter, monkeypatch):
+    """Free-response channels must NOT auto-create threads — bot replies inline.
+
+    Without this, every message in a free-response channel would spin off a
+    thread (since the channel bypasses the @mention gate), defeating the
+    lightweight-chat purpose of free-response mode.
+    """
+    monkeypatch.setenv("DISCORD_REQUIRE_MENTION", "true")
+    monkeypatch.setenv("DISCORD_FREE_RESPONSE_CHANNELS", "789")
+    monkeypatch.delenv("DISCORD_AUTO_THREAD", raising=False)  # default true
+
+    adapter._auto_create_thread = AsyncMock()
+
+    message = make_message(
+        channel=FakeTextChannel(channel_id=789),
+        content="free chat message",
+    )
+
+    await adapter._handle_message(message)
+
+    adapter._auto_create_thread.assert_not_awaited()
+    adapter.handle_message.assert_awaited_once()
+    event = adapter.handle_message.await_args.args[0]
+    assert event.source.chat_type == "group"
+
+
 @pytest.mark.asyncio
 async def test_discord_voice_linked_parent_thread_still_requires_mention(adapter, monkeypatch):
     """Threads under a voice-linked channel should still require @mention."""