feat(whatsapp): native media sending — images, videos, documents

Add a /send-media endpoint to the WhatsApp bridge and corresponding
adapter methods so the agent can send files as native WhatsApp
attachments instead of plain-text URLs/paths.

- bridge.js: new POST /send-media endpoint using Baileys' native
  image/video/document/audio message types with MIME detection
- base.py: add send_video(), send_document(), send_image_file()
  with text fallbacks; route MEDIA: tags by file extension instead
  of always treating them as voice messages
- whatsapp.py: implement all media methods via a shared
  _send_media_to_bridge() helper; override send_image() to download
  URLs to local cache and send as native photos
- prompt_builder.py: update WhatsApp and Telegram platform hints so
  the agent knows it can use MEDIA:/path tags to send native media

gateway/platforms/whatsapp.py

@@ -281,7 +281,102 @@ class WhatsAppAdapter(BasePlatformAdapter):
             )
         except Exception as e:
             return SendResult(success=False, error=str(e))
-    
+
+    async def _send_media_to_bridge(
+        self,
+        chat_id: str,
+        file_path: str,
+        media_type: str,
+        caption: Optional[str] = None,
+        file_name: Optional[str] = None,
+    ) -> SendResult:
+        """Send any media file via bridge /send-media endpoint."""
+        if not self._running:
+            return SendResult(success=False, error="Not connected")
+        try:
+            import aiohttp
+
+            if not os.path.exists(file_path):
+                return SendResult(success=False, error=f"File not found: {file_path}")
+
+            payload: Dict[str, Any] = {
+                "chatId": chat_id,
+                "filePath": file_path,
+                "mediaType": media_type,
+            }
+            if caption:
+                payload["caption"] = caption
+            if file_name:
+                payload["fileName"] = file_name
+
+            async with aiohttp.ClientSession() as session:
+                async with session.post(
+                    f"http://localhost:{self._bridge_port}/send-media",
+                    json=payload,
+                    timeout=aiohttp.ClientTimeout(total=120),
+                ) as resp:
+                    if resp.status == 200:
+                        data = await resp.json()
+                        return SendResult(
+                            success=True,
+                            message_id=data.get("messageId"),
+                            raw_response=data,
+                        )
+                    else:
+                        error = await resp.text()
+                        return SendResult(success=False, error=error)
+
+        except Exception as e:
+            return SendResult(success=False, error=str(e))
+
+    async def send_image(
+        self,
+        chat_id: str,
+        image_url: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+    ) -> SendResult:
+        """Download image URL to cache, send natively via bridge."""
+        try:
+            local_path = await cache_image_from_url(image_url)
+            return await self._send_media_to_bridge(chat_id, local_path, "image", caption)
+        except Exception:
+            return await super().send_image(chat_id, image_url, caption, reply_to)
+
+    async def send_image_file(
+        self,
+        chat_id: str,
+        image_path: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+    ) -> SendResult:
+        """Send a local image file natively via bridge."""
+        return await self._send_media_to_bridge(chat_id, image_path, "image", caption)
+
+    async def send_video(
+        self,
+        chat_id: str,
+        video_path: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+    ) -> SendResult:
+        """Send a video natively via bridge — plays inline in WhatsApp."""
+        return await self._send_media_to_bridge(chat_id, video_path, "video", caption)
+
+    async def send_document(
+        self,
+        chat_id: str,
+        file_path: str,
+        caption: Optional[str] = None,
+        file_name: Optional[str] = None,
+        reply_to: Optional[str] = None,
+    ) -> SendResult:
+        """Send a document/file as a downloadable attachment via bridge."""
+        return await self._send_media_to_bridge(
+            chat_id, file_path, "document", caption,
+            file_name or os.path.basename(file_path),
+        )
+
     async def send_typing(self, chat_id: str) -> None:
         """Send typing indicator via bridge."""
         if not self._running: