Add samePhoneResponsePrefix config option

Automatically prefixes responses with a configurable string when in
same-phone mode. This helps distinguish bot replies from user messages
in the same chat bubble.

Example config:
  "samePhoneResponsePrefix": "🦞"

Will prefix all same-phone replies with the lobster emoji.

src/web/auto-reply.ts

@@ -594,6 +594,15 @@ export async function monitorWebProvider(
           );
           return;
         }
+        // Apply same-phone response prefix if configured and in same-phone mode
+        const samePhoneResponsePrefix = cfg.inbound?.samePhoneResponsePrefix;
+        if (isSamePhoneMode && samePhoneResponsePrefix && replyResult.text) {
+          // Only add prefix if not already present
+          if (!replyResult.text.startsWith(samePhoneResponsePrefix)) {
+            replyResult.text = `${samePhoneResponsePrefix} ${replyResult.text}`;
+          }
+        }
+
         try {
           await deliverWebReply({
             replyResult,