fix: concatenate system prompt instead of push to preserve agent prompts

When using custom OpenCode agents (defined in ~/.config/opencode/agents/),
output.system.push() creates a second element in the system[] array.
OpenCode maps each element to a separate {role: 'system'} message,
and providers only respect the last one — which drops the agent prompt.

Fix: concatenate DCP prompt to system[0] instead of pushing a new element.

Tested: isolated A/B test confirmed push() breaks agent prompt,
concatenation preserves it.

Author: Evgeniy

lib/hooks.ts

@@ -92,7 +92,15 @@ export function createSystemPromptHandler(
             return
         }
 
-        output.system.push(renderSystemPrompt(flags))
+        // Concatenate to system[0] instead of push() to avoid creating
+        // multiple {role: "system"} messages. OpenCode maps each system[]
+        // element to a separate system message, and providers typically
+        // only respect the last one — which drops the agent prompt.
+        if (output.system.length > 0) {
+            output.system[0] += "\n\n" + renderSystemPrompt(flags)
+        } else {
+            output.system.push(renderSystemPrompt(flags))
+        }
     }
 }