OpenCode runs JS/TS plugins, not shell hooks, so it needs a plugin of its own in plugin/opencode/. That plugin would call bin/hook.py with OpenCode's events and push the answer into experimental.chat.system.transform (the opening) or a tool.execute.after result (the reminder). OpenCode 1.19.3's plugin API is @opencode-ai/plugin 1.x. dev carries an unreleased 2.x API (define({id, setup}) with session.hook), which a 1.x plugin would need porting to.
Build it on whichever API is released by then, and add it to plugin/tests/e2e_harnesses.py: OpenCode takes an OpenAI-compatible provider, so the stand-in model there works for it too. Kilo Code, built on OpenCode, loads the same plugins from .kilo/plugin/.