fix(server): tell agents how to nest fenced code inside code blocks

apps/server/src/provider/RuntimeInstructions.test.ts

@@ -25,4 +25,10 @@ describe("buildRuntimeInstructions", () => {
     expect(instructions).toContain("through the Cursor harness.");
     expect(instructions).not.toContain("reasoning effort");
   });
+
+  it("tells the agent how to nest fenced code inside a code block", () => {
+    expect(buildRuntimeInstructions({ harness: "Claude Code" })).toContain(
+      "A code block that contains triple backticks needs a longer fence (four backticks or ~~~).",
+    );
+  });
 });

apps/server/src/provider/RuntimeInstructions.ts

@@ -13,7 +13,7 @@ export function buildRuntimeInstructions(runtime: {
   const effort = toSingleLine(runtime.reasoningEffort ?? "");
   const modelInfo = model && model !== "auto" && model !== "default" ? `, as ${model}` : "";
   const effortInfo = effort ? ` with ${effort} reasoning effort` : "";
-  return `<runtime_info>In case you're asked: you are running in T3 Code through the ${harness} harness${modelInfo}${effortInfo}. No need to mention this otherwise. You can embed images and videos in your response using Markdown with absolute file paths.</runtime_info>\n\n${PULL_REQUEST_LINKING_INSTRUCTIONS}`;
+  return `<runtime_info>In case you're asked: you are running in T3 Code through the ${harness} harness${modelInfo}${effortInfo}. No need to mention this otherwise. You can embed images and videos in your response using Markdown with absolute file paths. A code block that contains triple backticks needs a longer fence (four backticks or ~~~).</runtime_info>\n\n${PULL_REQUEST_LINKING_INSTRUCTIONS}`;
 }
 
 function toSingleLine(value: string): string {