feat(whatsapp): make reply prefix configurable via config.yaml

Reworked from PR #1764 (ifrederico) to use config.yaml instead of .env.

The WhatsApp bridge prepends a header to every outgoing message.
This was hardcoded to '⚕ *Hermes Agent*'. Users can now customize
or disable it via config.yaml:

  whatsapp:
    reply_prefix: ''                     # disable header
    reply_prefix: '🤖 *My Bot*\n───\n'  # custom prefix

How it works:
- load_gateway_config() reads whatsapp.reply_prefix from config.yaml
  and stores it in PlatformConfig.extra['reply_prefix']
- WhatsAppAdapter reads it from config.extra at init
- When spawning bridge.js, the adapter passes it as
  WHATSAPP_REPLY_PREFIX in the subprocess environment
- bridge.js handles undefined (default), empty (no header),
  or custom values with \\n escape support
- Self-chat echo suppression uses the configured prefix

Also fixes _config_version: was 9 but ENV_VARS_BY_VERSION had a
key 10 (TAVILY_API_KEY), so existing users at v9 would never be
prompted for Tavily. Bumped to 10 to close the gap. Added a
regression test to prevent this from happening again.

Credit: ifrederico (PR #1764) for the bridge.js implementation
and the config version gap discovery.

website/docs/user-guide/messaging/whatsapp.md

@@ -140,7 +140,14 @@ Hermes supports voice on WhatsApp:
 
 - **Incoming:** Voice messages (`.ogg` opus) are automatically transcribed using the configured STT provider: local `faster-whisper`, Groq Whisper (`GROQ_API_KEY`), or OpenAI Whisper (`VOICE_TOOLS_OPENAI_KEY`)
 - **Outgoing:** TTS responses are sent as MP3 audio file attachments
-- Agent responses are prefixed with "⚕ **Hermes Agent**" for easy identification
+- Agent responses are prefixed with "⚕ **Hermes Agent**" by default. You can customize or disable this in `config.yaml`:
+
+```yaml
+# ~/.hermes/config.yaml
+whatsapp:
+  reply_prefix: ""                          # Empty string disables the header
+  # reply_prefix: "🤖 *My Bot*\n──────\n"  # Custom prefix (supports \n for newlines)
+```
 
 ---