feat: implement edit_message() for Telegram/Discord/Slack and fix fallback regression

Building on PR #288's edit_message() abstraction:

- Telegram: edit_message_text() with MarkdownV2 + plain text fallback
- Discord: channel.fetch_message() + msg.edit() with length capping
- Slack: chat_update() via slack_bolt client

Also fixes the fallback regression in send_progress_messages() where
platforms that don't support editing would receive duplicated accumulated
tool lines. Now uses a can_edit flag — after the first failed edit, falls
back to sending individual lines (matching pre-PR behavior).

gateway/platforms/discord.py

@@ -206,7 +206,29 @@ class DiscordAdapter(BasePlatformAdapter):
             
         except Exception as e:
             return SendResult(success=False, error=str(e))
-    
+
+    async def edit_message(
+        self,
+        chat_id: str,
+        message_id: str,
+        content: str,
+    ) -> SendResult:
+        """Edit a previously sent Discord message."""
+        if not self._client:
+            return SendResult(success=False, error="Not connected")
+        try:
+            channel = self._client.get_channel(int(chat_id))
+            if not channel:
+                channel = await self._client.fetch_channel(int(chat_id))
+            msg = await channel.fetch_message(int(message_id))
+            formatted = self.format_message(content)
+            if len(formatted) > self.MAX_MESSAGE_LENGTH:
+                formatted = formatted[:self.MAX_MESSAGE_LENGTH - 3] + "..."
+            await msg.edit(content=formatted)
+            return SendResult(success=True, message_id=message_id)
+        except Exception as e:
+            return SendResult(success=False, error=str(e))
+
     async def send_voice(
         self,
         chat_id: str,

gateway/platforms/slack.py

@@ -156,6 +156,25 @@ class SlackAdapter(BasePlatformAdapter):
             print(f"[Slack] Send error: {e}")
             return SendResult(success=False, error=str(e))
 
+    async def edit_message(
+        self,
+        chat_id: str,
+        message_id: str,
+        content: str,
+    ) -> SendResult:
+        """Edit a previously sent Slack message."""
+        if not self._app:
+            return SendResult(success=False, error="Not connected")
+        try:
+            await self._app.client.chat_update(
+                channel=chat_id,
+                ts=message_id,
+                text=content,
+            )
+            return SendResult(success=True, message_id=message_id)
+        except Exception as e:
+            return SendResult(success=False, error=str(e))
+
     async def send_typing(self, chat_id: str) -> None:
         """Slack doesn't have a direct typing indicator API for bots."""
         pass

gateway/platforms/telegram.py

@@ -218,7 +218,36 @@ class TelegramAdapter(BasePlatformAdapter):
             
         except Exception as e:
             return SendResult(success=False, error=str(e))
-    
+
+    async def edit_message(
+        self,
+        chat_id: str,
+        message_id: str,
+        content: str,
+    ) -> SendResult:
+        """Edit a previously sent Telegram message."""
+        if not self._bot:
+            return SendResult(success=False, error="Not connected")
+        try:
+            formatted = self.format_message(content)
+            try:
+                await self._bot.edit_message_text(
+                    chat_id=int(chat_id),
+                    message_id=int(message_id),
+                    text=formatted,
+                    parse_mode=ParseMode.MARKDOWN_V2,
+                )
+            except Exception:
+                # Fallback: retry without markdown formatting
+                await self._bot.edit_message_text(
+                    chat_id=int(chat_id),
+                    message_id=int(message_id),
+                    text=content,
+                )
+            return SendResult(success=True, message_id=message_id)
+        except Exception as e:
+            return SendResult(success=False, error=str(e))
+
     async def send_voice(
         self,
         chat_id: str,