[Feature Request] Add opencode support

[gkzhb]

I would like to suggest opencode, which is an Open Source alternative to Claude Code.

It provides plenty of customization features(like LSP, custom LLM providers, custom tools) and we can use HTTP server APIs to control opencode.

For emacs interface, I see there is context command feature in neovim plugin NickvanDyke/opencode.nvim which is quite handy for my daily usage.
For example, @cursor in the prompt will be replaced by the file cursor position and then sent to Coding Agent.
Users can also script their own commands to inject specific context into prompt.

[tninja]

Thanks for trying this library. I haven't try opencode yet. But seems it is under active development. This PR add the github-copilot-cli (require claude-code.el). Maybe adding opencode support is similar to this. Thanks for sharing HTTP server APIs, it could improve integration between emacs to the AI coding tool.

I like idea about providing more context. This tool currently have part of it automatically turned on. Eg. @buffer, @Buffers (all buffers in current window). Visible text might be a good to have. I feel Claude Code and other similar tools understand line numbers, maybe AI can understand things like: the visible region: line x to line y