feat: add IM channels for Feishu, Slack, and Telegram (#1010)

* feat: add IM channels system for Feishu, Slack, and Telegram integration

Bridge external messaging platforms to DeerFlow via LangGraph Server with
async message bus, thread management, and per-channel configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address review comments on IM channels system

Fix topic_id handling in store remove/list_entries and manager commands,
correct Telegram reply threading, remove unused imports/variables, update
docstrings and docs to match implementation, and prevent config mutation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* update skill creator

* fix im reply text

* fix comments

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/channels/base.py

@@ -0,0 +1,88 @@
+"""Abstract base class for IM channels."""
+
+from __future__ import annotations
+
+import logging
+from abc import ABC, abstractmethod
+from typing import Any
+
+from src.channels.message_bus import InboundMessage, InboundMessageType, MessageBus, OutboundMessage
+
+logger = logging.getLogger(__name__)
+
+
+class Channel(ABC):
+    """Base class for all IM channel implementations.
+
+    Each channel connects to an external messaging platform and:
+    1. Receives messages, wraps them as InboundMessage, publishes to the bus.
+    2. Subscribes to outbound messages and sends replies back to the platform.
+
+    Subclasses must implement ``start``, ``stop``, and ``send``.
+    """
+
+    def __init__(self, name: str, bus: MessageBus, config: dict[str, Any]) -> None:
+        self.name = name
+        self.bus = bus
+        self.config = config
+        self._running = False
+
+    @property
+    def is_running(self) -> bool:
+        return self._running
+
+    # -- lifecycle ---------------------------------------------------------
+
+    @abstractmethod
+    async def start(self) -> None:
+        """Start listening for messages from the external platform."""
+
+    @abstractmethod
+    async def stop(self) -> None:
+        """Gracefully stop the channel."""
+
+    # -- outbound ----------------------------------------------------------
+
+    @abstractmethod
+    async def send(self, msg: OutboundMessage) -> None:
+        """Send a message back to the external platform.
+
+        The implementation should use ``msg.chat_id`` and ``msg.thread_ts``
+        to route the reply to the correct conversation/thread.
+        """
+
+    # -- helpers -----------------------------------------------------------
+
+    def _make_inbound(
+        self,
+        chat_id: str,
+        user_id: str,
+        text: str,
+        *,
+        msg_type: InboundMessageType = InboundMessageType.CHAT,
+        thread_ts: str | None = None,
+        files: list[dict[str, Any]] | None = None,
+        metadata: dict[str, Any] | None = None,
+    ) -> InboundMessage:
+        """Convenience factory for creating InboundMessage instances."""
+        return InboundMessage(
+            channel_name=self.name,
+            chat_id=chat_id,
+            user_id=user_id,
+            text=text,
+            msg_type=msg_type,
+            thread_ts=thread_ts,
+            files=files or [],
+            metadata=metadata or {},
+        )
+
+    async def _on_outbound(self, msg: OutboundMessage) -> None:
+        """Outbound callback registered with the bus.
+
+        Only forwards messages targeted at this channel.
+        """
+        if msg.channel_name == self.name:
+            try:
+                await self.send(msg)
+            except Exception:
+                logger.exception("Failed to send outbound message on channel %s", self.name)