Fix OpenCode plugin Jinja template corruption with Qwen3.6

[atomicflag]

Issue

The OpenCode plugin's system.transform hook was corrupting OpenCode's Jinja template when injecting planning prompts. The error appeared as:

raise_exception('System message must be at the beginning.')

This occurred when using Qwen3.6 models with their chat template.

Root Cause

The plugin was pushing planning prompts and improvement context as separate elements in the output.system array:

output.system.push(getPlanningPrompt());
output.system.push(improveContext);

OpenCode maps each element of output.system to a separate system message. The Qwen3.6 Jinja template only allows one system message at position 0:

{%- for message in messages %}
    {%- if message.role == "system" %}
        {%- if not loop.first %}
            {{- raise_exception('System message must be at the beginning.') }}
        {%- endif %}
    {%- endif %}
{%- endfor %}

When the plugin pushed multiple elements, OpenCode created multiple system messages, causing the template to fail at position > 0.

Jinja template reference: https://huggingface.co/Qwen/Qwen3.6-35B-A3B-FP8/raw/main/chat_template.jinja

Solution

Instead of pushing new elements to output.system, the plugin now appends content to the first element (output.system[0]):

output.system[0] += "\n\n" + getPlanningPrompt();

This preserves the multi-message output.system array structure while keeping all injected content within the original system message (element 0). The Jinja template sees only one system message at position 0, containing all the plugin's additions.

Testing

• Verified the fix works with Qwen3.6 models
• All existing tests pass (10/10 in plan-mode.test.ts)
• No breaking changes to other agents or models