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
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