fix: when using codex sub, send the custom agent prompts as a separate developer message (previously sent as user message but api allows for instructions AND developer messages) (#11667)

Co-authored-by: Carlos <carloscanas942@gmail.com>

packages/opencode/src/session/llm.ts

@@ -233,19 +233,12 @@ export namespace LLM {
       },
       maxRetries: input.retries ?? 0,
       messages: [
-        ...(isCodex
-          ? [
-              {
-                role: "user",
-                content: system.join("\n\n"),
-              } as ModelMessage,
-            ]
-          : system.map(
-              (x): ModelMessage => ({
-                role: "system",
-                content: x,
-              }),
-            )),
+        ...system.map(
+          (x): ModelMessage => ({
+            role: "system",
+            content: x,
+          }),
+        ),
         ...input.messages,
       ],
       model: wrapLanguageModel({