The ACP runtime handed the classifier's system prompt to hive-runtime's
system_prompt_file, which prepends it to the first user message, so the
instructions and the PR text sat in the same user turn.
The rendered opencode.json now defines a review-classifier agent whose
prompt is {file:./system.md} and selects it with default_agent. opencode
1.15.10 sends an agent's prompt as the system-role message (in place of
its per-model default prompt), followed only by its environment block;
the first user message is the PR material alone. The API key runtime
keeps --system-prompt.
The agent carries no permission or tools key, and a test pins its exact
shape: opencode merges an agent's own permission after
OPENCODE_PERMISSION, so a permission there would offer tools again
(checked on real opencode: {"*":"allow"} on the agent offered all ten
tools despite OPENCODE_PERMISSION={"*":"deny"}).
Follow-up to #5214.
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