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).

agent/prompt_builder.py

@@ -103,12 +103,24 @@ PLATFORM_HINTS = {
         "You are on a text messaging communication platform, Telegram. "
         "Please do not use markdown as it does not render. "
         "You can send media files natively: to deliver a file to the user, "
-        "include MEDIA:/absolute/path/to/file in your response. Audio "
-        "(.ogg) sends as voice bubbles. You can also include image URLs "
-        "in markdown format ![alt](url) and they will be sent as native photos."
+        "include MEDIA:/absolute/path/to/file in your response. Images "
+        "(.png, .jpg, .webp) appear as photos, audio (.ogg) sends as voice "
+        "bubbles, and videos (.mp4) play inline. You can also include image "
+        "URLs in markdown format ![alt](url) and they will be sent as native photos."
     ),
     "discord": (
-        "You are in a Discord server or group chat communicating with your user."
+        "You are in a Discord server or group chat communicating with your user. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.png, .jpg, .webp) are sent as photo "
+        "attachments, audio as file attachments. You can also include image URLs "
+        "in markdown format ![alt](url) and they will be sent as attachments."
+    ),
+    "slack": (
+        "You are in a Slack workspace communicating with your user. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.png, .jpg, .webp) are uploaded as photo "
+        "attachments, audio as file attachments. You can also include image URLs "
+        "in markdown format ![alt](url) and they will be uploaded as attachments."
     ),
     "cli": (
         "You are a CLI AI Agent. Try not to use markdown but simple text "