fix(channel): coalesce queued stream deltas to reduce API calls

When LLM generates faster than channel can process, asyncio.Queue
accumulates multiple _stream_delta messages. Each delta triggers a
separate API call (~700ms each), causing visible delay after LLM
finishes.

Solution: In _dispatch_outbound, drain all queued deltas for the same
(channel, chat_id) before sending, combining them into a single API
call. Non-matching messages are preserved in a pending buffer for
subsequent processing.

This reduces N API calls to 1 when queue has N accumulated deltas.

nanobot/channels/manager.py

@@ -118,12 +118,20 @@ class ChannelManager:
         """Dispatch outbound messages to the appropriate channel."""
         logger.info("Outbound dispatcher started")
 
+        # Buffer for messages that couldn't be processed during delta coalescing
+        # (since asyncio.Queue doesn't support push_front)
+        pending: list[OutboundMessage] = []
+
         while True:
             try:
-                msg = await asyncio.wait_for(
-                    self.bus.consume_outbound(),
-                    timeout=1.0
-                )
+                # First check pending buffer before waiting on queue
+                if pending:
+                    msg = pending.pop(0)
+                else:
+                    msg = await asyncio.wait_for(
+                        self.bus.consume_outbound(),
+                        timeout=1.0
+                    )
 
                 if msg.metadata.get("_progress"):
                     if msg.metadata.get("_tool_hint") and not self.config.channels.send_tool_hints:
@@ -131,6 +139,12 @@ class ChannelManager:
                     if not msg.metadata.get("_tool_hint") and not self.config.channels.send_progress:
                         continue
 
+                # Coalesce consecutive _stream_delta messages for the same (channel, chat_id)
+                # to reduce API calls and improve streaming latency
+                if msg.metadata.get("_stream_delta") and not msg.metadata.get("_stream_end"):
+                    msg, extra_pending = self._coalesce_stream_deltas(msg)
+                    pending.extend(extra_pending)
+
                 channel = self.channels.get(msg.channel)
                 if channel:
                     await self._send_with_retry(channel, msg)
@@ -150,6 +164,54 @@ class ChannelManager:
         elif not msg.metadata.get("_streamed"):
             await channel.send(msg)
 
+    def _coalesce_stream_deltas(
+        self, first_msg: OutboundMessage
+    ) -> tuple[OutboundMessage, list[OutboundMessage]]:
+        """Merge consecutive _stream_delta messages for the same (channel, chat_id).
+
+        This reduces the number of API calls when the queue has accumulated multiple
+        deltas, which happens when LLM generates faster than the channel can process.
+
+        Returns:
+            tuple of (merged_message, list_of_non_matching_messages)
+        """
+        target_key = (first_msg.channel, first_msg.chat_id)
+        combined_content = first_msg.content
+        final_metadata = dict(first_msg.metadata or {})
+        non_matching: list[OutboundMessage] = []
+
+        # Drain all pending _stream_delta messages for the same (channel, chat_id)
+        while True:
+            try:
+                next_msg = self.bus.outbound.get_nowait()
+            except asyncio.QueueEmpty:
+                break
+
+            # Check if this message belongs to the same stream
+            same_target = (next_msg.channel, next_msg.chat_id) == target_key
+            is_delta = next_msg.metadata and next_msg.metadata.get("_stream_delta")
+            is_end = next_msg.metadata and next_msg.metadata.get("_stream_end")
+
+            if same_target and is_delta and not final_metadata.get("_stream_end"):
+                # Accumulate content
+                combined_content += next_msg.content
+                # If we see _stream_end, remember it and stop coalescing this stream
+                if is_end:
+                    final_metadata["_stream_end"] = True
+                    # Stream ended - stop coalescing this stream
+                    break
+            else:
+                # Keep for later processing
+                non_matching.append(next_msg)
+
+        merged = OutboundMessage(
+            channel=first_msg.channel,
+            chat_id=first_msg.chat_id,
+            content=combined_content,
+            metadata=final_metadata,
+        )
+        return merged, non_matching
+
     async def _send_with_retry(self, channel: BaseChannel, msg: OutboundMessage) -> None:
         """Send a message with retry on failure using exponential backoff.