feat(channels): use plain text as default output formatter for Signal

crates/openfang-channels/src/bridge.rs

@@ -517,6 +517,7 @@ fn default_output_format_for_channel(channel_type: &str) -> OutputFormat {
         "telegram" => OutputFormat::TelegramHtml,
         "slack" => OutputFormat::SlackMrkdwn,
         "wecom" => OutputFormat::PlainText,
+        "signal" => OutputFormat::PlainText,
         _ => OutputFormat::Markdown,
     }
 }