debug(deploy): truncate _debug.txt per run for unambiguous diagnostics

scripts/deploy-via-docker.js

@@ -179,6 +179,21 @@ async function main() {
   }
   dbg('helper started');
 
+  // Truncate the host debug file so each run starts clean (job container itself
+  // cannot write here; use the helper we just started).
+  const trunc = await dockerJson('POST', `/containers/${HELPER_ID}/exec`, {
+    Cmd: ['sh', '-c', `rm -f ${HOST_STATIC}/_debug.txt`],
+    AttachStdin: false,
+    AttachStdout: false,
+    AttachStderr: false,
+  });
+  if (trunc.status < 400) {
+    const tId = JSON.parse(trunc.body).Id;
+    await dockerJson('POST', `/exec/${tId}/start`, { Detach: true, Tty: false });
+    await new Promise((r) => setTimeout(r, 200));
+  }
+  dbg('debug file truncated for this run');
+
   // 2) locate the built assets inside THIS container.
   const jobId = getContainerId();
   dbg('job container id=' + jobId);