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/slack.py

@@ -179,6 +179,35 @@ class SlackAdapter(BasePlatformAdapter):
         """Slack doesn't have a direct typing indicator API for bots."""
         pass
 
+    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 to Slack by uploading it."""
+        if not self._app:
+            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}")
+
+            result = await self._app.client.files_upload_v2(
+                channel=chat_id,
+                file=image_path,
+                filename=os.path.basename(image_path),
+                initial_comment=caption or "",
+                thread_ts=reply_to,
+            )
+            return SendResult(success=True, raw_response=result)
+
+        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,