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>

src/jarvis/tools/registry.py

@@ -21,6 +21,7 @@ from .builtin.weather import WeatherTool
 from .builtin.stop import StopTool
 from .builtin.tool_search import ToolSearchTool
 from .builtin.browse_and_play import BrowseAndPlayTool
+from .builtin.set_broadcast import SetBroadcastTool
 from .types import ToolExecutionResult
 from ..config import Settings
 from .external.mcp_client import MCPClient
@@ -41,6 +42,7 @@ BUILTIN_TOOLS = {
     "stop": StopTool(),
     "toolSearchTool": ToolSearchTool(),
     "browseAndPlay": BrowseAndPlayTool(),
+    "setBroadcast": SetBroadcastTool(),
 }
 
 # Global MCP tools cache