Stop inviting markdown image syntax for local files

The hint said image markdown gets uploaded as an attachment without
saying that only applies to http(s). The model followed it for a local
file, and gateway's extract_images matches https?:// only — so
![alt](file:///…/squirrel.jpg) was never extracted and got posted as
literal text while the agent believed it had delivered the image.

Spell out that local files always go through MEDIA:, the way the built-in
WebUI hint already does for the same reason.

adapter.py

@@ -2109,15 +2109,16 @@ def register(ctx) -> None:
         allow_update_command=True,
         pii_safe=False,
         platform_hint=(
-            "You are chatting in Chatto (a self-hosted or cloud-hosted team or community chat server). "
-            "Markdown is supported. Users _may_ address you by @-mentioning your name. If configured, " \
+            "Using the 'Hermes Chatto Platform Plugin' you connect to a Chatto server. "
+            "Authorized admins and users will contact you and call you his Hermes Agent. They are "
+            "natural persons and thus responsible for what they do in terms of rights. You compute "
+            "on their behalf. They _may_ address you by @-mentioning your name. If configured, "
             "you also react without a @-mention. Direct messages reach you without a mention."
-            "Keep responses conversational. "
-            "You can send media files natively — never shell out to a CLI to deliver a file. "
-            "To send a local file, include MEDIA:/absolute/path/to/file in your response: images "
+            "Keep responses conversational. Markdown is supported. "
+            "Include MEDIA:/absolute/path/to/file in your response to refer to our local files. Images "
             "(.png, .jpg, .gif, .webp) arrive as inline pictures, videos (.mp4, .mov, .webm) as "
             "video attachments, audio as a voice bubble, anything else as a downloadable document. "
             "Do NOT use markdown image syntax for local files. Local files always go through MEDIA:. "
-            "Several images in one response are bundled into a single message."
+            "Several images in one response are bundled into a single message. "
         ),
     )