fix: merge system messages for llama.cpp compatibility

[winmutt]

Problem

Using the opencode-agent-memory plugin with Qwen 3.5 via llama.cpp results in:

Error: Jinja Exception: System message must be at the beginning.

Root Cause

The experimental.chat.system.transform hook adds memory blocks and journal instructions to system messages without merging pre-existing system messages. When the provider already has multiple system messages, the plugin leaves them as multiple messages, which violates llama.cpp Jinja template requirements.

Solution

Merge all system messages (memory blocks, provider content, journal instructions) into a single system message at index 0.

Testing

Added unit tests in src/plugin.test.ts to verify:

• Multiple system messages are merged into one
• Empty system arrays are handled
• Memory blocks are prepended before provider content