fix(discord): voice session continuity and signal handler thread safety

- Store source metadata on /voice channel join so voice input shares the
  same session as the linked text channel conversation
- Treat voice-linked text channels as free-response (skip @mention and
  auto-thread) while voice is active
- Scope the voice-linked exemption to the exact bound channel, not
  sibling threads
- Guard signal handler registration in start_gateway() for non-main
  threads (prevents RuntimeError when gateway runs in a daemon thread)
- Clean up _voice_sources on leave_voice_channel

Salvaged from PR #3475 by twilwa (Modal runtime portions excluded).

gateway/run.py

@@ -4927,6 +4927,8 @@ class GatewayRunner:
 
         if success:
             adapter._voice_text_channels[guild_id] = int(event.source.chat_id)
+            if hasattr(adapter, "_voice_sources"):
+                adapter._voice_sources[guild_id] = event.source.to_dict()
             self._voice_mode[event.source.chat_id] = "all"
             self._save_voice_modes()
             self._set_adapter_auto_tts_disabled(adapter, event.source.chat_id, disabled=False)
@@ -4987,14 +4989,23 @@ class GatewayRunner:
         if not text_ch_id:
             return
 
+        # Build source — reuse the linked text channel's metadata when available
+        # so voice input shares the same session as the bound text conversation.
+        source_data = getattr(adapter, "_voice_sources", {}).get(guild_id)
+        if source_data:
+            source = SessionSource.from_dict(source_data)
+            source.user_id = str(user_id)
+            source.user_name = str(user_id)
+        else:
+            source = SessionSource(
+                platform=Platform.DISCORD,
+                chat_id=str(text_ch_id),
+                user_id=str(user_id),
+                user_name=str(user_id),
+                chat_type="channel",
+            )
+
         # Check authorization before processing voice input
-        source = SessionSource(
-            platform=Platform.DISCORD,
-            chat_id=str(text_ch_id),
-            user_id=str(user_id),
-            user_name=str(user_id),
-            chat_type="channel",
-        )
         if not self._is_user_authorized(source):
             logger.debug("Unauthorized voice input from user %d, ignoring", user_id)
             return
@@ -8891,16 +8902,19 @@ async def start_gateway(config: Optional[GatewayConfig] = None, replace: bool =
         runner.request_restart(detached=False, via_service=True)
     
     loop = asyncio.get_event_loop()
-    for sig in (signal.SIGINT, signal.SIGTERM):
-        try:
-            loop.add_signal_handler(sig, shutdown_signal_handler)
-        except NotImplementedError:
-            pass
-    if hasattr(signal, "SIGUSR1"):
-        try:
-            loop.add_signal_handler(signal.SIGUSR1, restart_signal_handler)
-        except NotImplementedError:
-            pass
+    if threading.current_thread() is threading.main_thread():
+        for sig in (signal.SIGINT, signal.SIGTERM):
+            try:
+                loop.add_signal_handler(sig, shutdown_signal_handler)
+            except NotImplementedError:
+                pass
+        if hasattr(signal, "SIGUSR1"):
+            try:
+                loop.add_signal_handler(signal.SIGUSR1, restart_signal_handler)
+            except NotImplementedError:
+                pass
+    else:
+        logger.info("Skipping signal handlers (not running in main thread).")
     
     # Start the gateway
     success = await runner.start()