Fix empty result JSON dump and missing heartbeat prefix

Bug fixes:
- Empty result field handling: Changed truthy check to explicit type
  check (`typeof parsed?.text === "string"`) in command-reply.ts.
  Previously, Claude CLI returning `result: ""` would cause raw JSON
  to be sent to WhatsApp.
- Response prefix on heartbeat: Apply `responsePrefix` to heartbeat
  alert messages in runReplyHeartbeat, matching behavior of regular
  message handler.

src/web/auto-reply.ts

@@ -862,9 +862,16 @@ export async function monitorWebProvider(
           return;
         }
 
+        // Apply response prefix if configured (same as regular messages)
+        let finalText = stripped.text;
+        const responsePrefix = cfg.inbound?.responsePrefix;
+        if (responsePrefix && finalText && !finalText.startsWith(responsePrefix)) {
+          finalText = `${responsePrefix} ${finalText}`;
+        }
+
         const cleanedReply: ReplyPayload = {
           ...replyResult,
-          text: stripped.text,
+          text: finalText,
         };
 
         await deliverWebReply({