The WhatsApp bridge prepends '⚕ *Hermes Agent*\n────────────\n' to
every outgoing message. In self-chat mode this is necessary to
distinguish the bot's responses from the user's own messages. In bot
mode the messages already come from a different number, making the
prefix redundant and cluttered.
Now only prepends the prefix when WHATSAPP_MODE is 'self-chat' (the
default). Bot mode messages are sent clean.
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