fix: improve apply-patch add-file guidance

scripts/ssh-argv-guidance-contract-test.ts

@@ -905,6 +905,24 @@ export async function runSshArgvGuidanceContract(): Promise<JsonRecord> {
   assertCondition(addBeforeFailedUpdateV2.files.hwpod?.includes("device-pod-cli.mjs"), "v2 should preserve an earlier Add File from a large patch when a later hunk misses", addBeforeFailedUpdateV2);
   assertCondition(addBeforeFailedUpdateV2.files["scripts/artifact-publish.mjs"] === "actual artifact line\n", "v2 must leave the failed later file unchanged", addBeforeFailedUpdateV2);
 
+  const addFileWithHunkMarkerV2 = await applyPatchV2FixtureAttempt([
+    "*** Begin Patch",
+    "*** Add File: bad-add.txt",
+    "@@",
+    "+content",
+    "*** End Patch",
+    "",
+  ].join("\n"), {}, { stderrOutput: true });
+  assertCondition(addFileWithHunkMarkerV2.exitCode === 1 && addFileWithHunkMarkerV2.error === null, "v2 CLI path should return a visible parse error for Add File @@ misuse", addFileWithHunkMarkerV2);
+  assertCondition(addFileWithHunkMarkerV2.stdout === "", "v2 Add File parse failures should keep Codex-style empty stdout", addFileWithHunkMarkerV2);
+  assertCondition(
+    addFileWithHunkMarkerV2.stderr.includes("remove @@ for Add File")
+      && addFileWithHunkMarkerV2.stderr.includes("prefix every content line with +"),
+    "v2 Add File @@ misuse should tell agents exactly how to repair the patch",
+    addFileWithHunkMarkerV2.stderr,
+  );
+  assertCondition(!addFileWithHunkMarkerV2.commands.some((command) => command.startsWith("write-b64") || command.startsWith("delete")), "v2 Add File parse failures must not touch remote files", addFileWithHunkMarkerV2.commands);
+
   const sequentialCompoundV2 = await applyPatchV2Fixture([
     "*** Begin Patch",
     "*** Update File: sequence.txt",