feat(gateway): streaming token delivery — StreamingConfig, GatewayStreamConsumer, already_sent

Stage 3 of streaming support. Gateway now streams tokens to messaging platforms:

- StreamingConfig dataclass (enabled, transport, edit_interval, buffer_threshold, cursor)
  on GatewayConfig with from_dict/to_dict serialization
- GatewayStreamConsumer: async queue-based consumer that progressively edits
  a single message on the target platform (edit transport)
- on_delta() → queue → run() async task → send_or_edit() with rate limiting
- already_sent propagation: when streaming delivered the response, handler
  returns None so base adapter skips duplicate send()
- stream_delta_callback wired into AIAgent constructor in _run_agent
- Consumer lifecycle: started as asyncio task, awaited with timeout in finally

Config (config.yaml):
  streaming:
    enabled: true
    transport: edit      # progressive editMessageText
    edit_interval: 0.3   # seconds between edits
    buffer_threshold: 40 # chars before forcing flush
    cursor: ' ▉'

Credit: jobless0x (#774, #1312), OutThisLife (#798), clicksingh (#697).

gateway/stream_consumer.py

@@ -0,0 +1,172 @@
+"""Gateway streaming consumer — bridges sync agent callbacks to async platform delivery.
+
+The agent fires stream_delta_callback(text) synchronously from its worker thread.
+GatewayStreamConsumer:
+  1. Receives deltas via on_delta() (thread-safe, sync)
+  2. Queues them to an asyncio task via queue.Queue
+  3. The async run() task buffers, rate-limits, and progressively edits
+     a single message on the target platform
+
+Design: Uses the edit transport (send initial message, then editMessageText).
+This is universally supported across Telegram, Discord, and Slack.
+
+Credit: jobless0x (#774, #1312), OutThisLife (#798), clicksingh (#697).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+import queue
+import time
+from dataclasses import dataclass
+from typing import Any, Optional
+
+logger = logging.getLogger("gateway.stream_consumer")
+
+# Sentinel to signal the stream is complete
+_DONE = object()
+
+
+@dataclass
+class StreamConsumerConfig:
+    """Runtime config for a single stream consumer instance."""
+    edit_interval: float = 0.3
+    buffer_threshold: int = 40
+    cursor: str = " ▉"
+
+
+class GatewayStreamConsumer:
+    """Async consumer that progressively edits a platform message with streamed tokens.
+
+    Usage::
+
+        consumer = GatewayStreamConsumer(adapter, chat_id, config, metadata=metadata)
+        # Pass consumer.on_delta as stream_delta_callback to AIAgent
+        agent = AIAgent(..., stream_delta_callback=consumer.on_delta)
+        # Start the consumer as an asyncio task
+        task = asyncio.create_task(consumer.run())
+        # ... run agent in thread pool ...
+        consumer.finish()  # signal completion
+        await task         # wait for final edit
+    """
+
+    def __init__(
+        self,
+        adapter: Any,
+        chat_id: str,
+        config: Optional[StreamConsumerConfig] = None,
+        metadata: Optional[dict] = None,
+    ):
+        self.adapter = adapter
+        self.chat_id = chat_id
+        self.cfg = config or StreamConsumerConfig()
+        self.metadata = metadata
+        self._queue: queue.Queue = queue.Queue()
+        self._accumulated = ""
+        self._message_id: Optional[str] = None
+        self._already_sent = False
+        self._last_edit_time = 0.0
+
+    @property
+    def already_sent(self) -> bool:
+        """True if at least one message was sent/edited — signals the base
+        adapter to skip re-sending the final response."""
+        return self._already_sent
+
+    def on_delta(self, text: str) -> None:
+        """Thread-safe callback — called from the agent's worker thread."""
+        if text:
+            self._queue.put(text)
+
+    def finish(self) -> None:
+        """Signal that the stream is complete."""
+        self._queue.put(_DONE)
+
+    async def run(self) -> None:
+        """Async task that drains the queue and edits the platform message."""
+        try:
+            while True:
+                # Drain all available items from the queue
+                got_done = False
+                while True:
+                    try:
+                        item = self._queue.get_nowait()
+                        if item is _DONE:
+                            got_done = True
+                            break
+                        self._accumulated += item
+                    except queue.Empty:
+                        break
+
+                # Decide whether to flush an edit
+                now = time.monotonic()
+                elapsed = now - self._last_edit_time
+                should_edit = (
+                    got_done
+                    or (elapsed >= self.cfg.edit_interval
+                        and len(self._accumulated) > 0)
+                    or len(self._accumulated) >= self.cfg.buffer_threshold
+                )
+
+                if should_edit and self._accumulated:
+                    display_text = self._accumulated
+                    if not got_done:
+                        display_text += self.cfg.cursor
+
+                    await self._send_or_edit(display_text)
+                    self._last_edit_time = time.monotonic()
+
+                if got_done:
+                    # Final edit without cursor
+                    if self._accumulated and self._message_id:
+                        await self._send_or_edit(self._accumulated)
+                    return
+
+                await asyncio.sleep(0.05)  # Small yield to not busy-loop
+
+        except asyncio.CancelledError:
+            # Best-effort final edit on cancellation
+            if self._accumulated and self._message_id:
+                try:
+                    await self._send_or_edit(self._accumulated)
+                except Exception:
+                    pass
+        except Exception as e:
+            logger.error("Stream consumer error: %s", e)
+
+    async def _send_or_edit(self, text: str) -> None:
+        """Send or edit the streaming message."""
+        try:
+            if self._message_id is not None:
+                # Edit existing message
+                result = await self.adapter.edit_message(
+                    chat_id=self.chat_id,
+                    message_id=self._message_id,
+                    content=text,
+                )
+                if result.success:
+                    self._already_sent = True
+                else:
+                    # Edit failed — try sending as new message
+                    logger.debug("Edit failed, sending new message")
+                    result = await self.adapter.send(
+                        chat_id=self.chat_id,
+                        content=text,
+                        metadata=self.metadata,
+                    )
+                    if result.success and result.message_id:
+                        self._message_id = result.message_id
+                        self._already_sent = True
+            else:
+                # First message — send new
+                result = await self.adapter.send(
+                    chat_id=self.chat_id,
+                    content=text,
+                    metadata=self.metadata,
+                )
+                if result.success and result.message_id:
+                    self._message_id = result.message_id
+                    self._already_sent = True
+        except Exception as e:
+            logger.error("Stream send/edit error: %s", e)