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/auto-reply/command-reply.ts

@@ -230,7 +230,7 @@ export async function runCommandReply(
           `Claude JSON raw: ${JSON.stringify(parsed.parsed, null, 2)}`,
         );
       }
-      if (parsed?.text) {
+      if (typeof parsed?.text === "string") {
         logVerbose(
           `Claude JSON parsed -> ${parsed.text.slice(0, 120)}${parsed.text.length > 120 ? "…" : ""}`,
         );