feat: couple broadcast to voice + voice-controlled broadcast toggle

Completes the STREAM_BROWSER=true behaviour:
- handleJoin auto-starts the broadcast on voice join and wires the session to
  the guild streamer; each turn reports the live state to the brain so search
  routes Chrome (live) vs Gemini (off).
- New setBroadcast tool lets the user toggle the broadcast by voice ("방송
  켜줘/꺼줘") via the LLM (no hardcoded phrases); it refuses when
  STREAM_BROWSER=false. The directive flows brain -> bridge (broadcast_action)
  -> bot streamer.start/stop, guarded by isActive() so it's idempotent.
- Per-turn IPC uses a thread-local (reply/turn_state.py) instead of threading
  params through the whole engine chain: bridge sets broadcasting in, tool
  records the directive out; Tool.execute exposes broadcasting on ToolContext.

Bot typecheck clean; brain covered by tests/test_set_broadcast.py (+ existing
routing tests). Specs + docs updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bot/src/index.ts

@@ -81,6 +81,30 @@ async function handleJoin(i: ChatInputCommandInteraction) {
   const session = await joinChannel(channel);
   session.onTurn = ({ transcript, reply }) =>
     console.log(`🗣️  ${transcript}\n🤖 ${reply}`);
+
+  // Screen-share mode (STREAM_BROWSER=true): the broadcast is coupled to the
+  // voice session. Auto-start it on join, report its live state to the brain
+  // each turn (search routes Chrome while live / Gemini when off), and let the
+  // brain toggle it via voice ("방송 켜줘 / 꺼줘"). In voice-only mode
+  // (STREAM_BROWSER=false) none of this runs and the broadcast stays off.
+  if (config.screenBrowser) {
+    const streamer = await getStreamer(i.guildId!);
+    const ctx: StreamContext = { guildId: i.guildId!, voiceChannelId: channel.id };
+    session.getBroadcasting = () => streamer.isActive();
+    session.onBroadcastAction = async (action) => {
+      if (action === "start") {
+        if (!streamer.isActive()) await streamer.start(ctx);
+      } else if (streamer.isActive()) {
+        await streamer.stop();
+      }
+    };
+    try {
+      if (!streamer.isActive()) await streamer.start(ctx);
+    } catch (e) {
+      console.error("[join] auto-broadcast failed:", e);
+    }
+  }
+
   await i.editReply(`🎙️ '${channel.name}' 채널에 접속했습니다. 말씀하세요.`);
 }