add directory to opencode system-prompt/instructions writing

[verdverm]

Makes the file location consistent with other harnesses, was not being picked up by opencode because it ended up in $HOME instead of $HOME/.config/opencode

[gemini-code-assist]

Code Review

This pull request updates the file path for AGENTS.md in the OpenCode harness to be located within a .config/opencode subdirectory. Review feedback points out that the os.WriteFile calls will fail if the new directory structure does not already exist and recommends using os.MkdirAll to create the path, while also suggesting the use of a helper method to avoid hardcoding the configuration directory name.