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

agent/prompt_builder.py

@@ -90,11 +90,23 @@ SKILLS_GUIDANCE = (
 PLATFORM_HINTS = {
     "whatsapp": (
         "You are on a text messaging communication platform, WhatsApp. "
-        "Please do not use markdown as it does not render."
+        "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. The file "
+        "will be sent as a native WhatsApp attachment — images (.jpg, .png, "
+        ".webp) appear as photos, videos (.mp4, .mov) play inline, and other "
+        "files arrive as downloadable documents. You can also include image "
+        "URLs in markdown format ![alt](url) and they will be sent as photos."
     ),
     "telegram": (
         "You are on a text messaging communication platform, Telegram. "
-        "Please do not use markdown as it does not render."
+        "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. Images "
+        "(.jpg, .png) appear as photos, videos (.mp4) play inline, audio "
+        "(.ogg) sends as voice bubbles, and other files as documents. 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."