feat: browser screenshot sharing via MEDIA: on all messaging platforms

browser_vision now saves screenshots persistently to ~/.hermes/browser_screenshots/
and returns the screenshot_path in its JSON response. The model can include
MEDIA:<path> in its response to share screenshots as native photos.

Changes:
- browser_tool.py: Save screenshots persistently, return screenshot_path,
  auto-cleanup files older than 24 hours, mkdir moved inside try/except
- telegram.py: Add send_image_file() — sends local images via bot.send_photo()
- discord.py: Add send_image_file() — sends local images via discord.File
- slack.py: Add send_image_file() — sends local images via files_upload_v2()
  (WhatsApp already had send_image_file — no changes needed)
- prompt_builder.py: Updated Telegram hint to list image extensions,
  added Discord and Slack MEDIA: platform hints
- browser.md: Document screenshot sharing and 24h cleanup
- send_file_integration_map.md: Updated to reflect send_image_file is now
  implemented on Telegram/Discord/Slack
- test_send_image_file.py: 19 tests covering MEDIA: .png extraction,
  send_image_file on all platforms, and screenshot cleanup

Partially addresses #466 (Phase 0: platform adapter gaps for send_image_file).

gateway/platforms/telegram.py

@@ -306,6 +306,34 @@ class TelegramAdapter(BasePlatformAdapter):
             print(f"[{self.name}] Failed to send voice/audio: {e}")
             return await super().send_voice(chat_id, audio_path, 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 as a Telegram photo."""
+        if not self._bot:
+            return SendResult(success=False, error="Not connected")
+        
+        try:
+            import os
+            if not os.path.exists(image_path):
+                return SendResult(success=False, error=f"Image file not found: {image_path}")
+            
+            with open(image_path, "rb") as image_file:
+                msg = await self._bot.send_photo(
+                    chat_id=int(chat_id),
+                    photo=image_file,
+                    caption=caption[:1024] if caption else None,
+                    reply_to_message_id=int(reply_to) if reply_to else None,
+                )
+            return SendResult(success=True, message_id=str(msg.message_id))
+        except Exception as e:
+            print(f"[{self.name}] Failed to send local image: {e}")
+            return await super().send_image_file(chat_id, image_path, caption, reply_to)
+
     async def send_image(
         self,
         chat_id: str,