services.hyperhive.agent.runtime ("claude" default | "acp") and
acp.{command,args,env}, rendered into HIVE_RUNTIME / HIVE_ACP_* only
for acp, so a claude agent's unit is unchanged. acp implies useApiKey.
acp.presets.opencode runs `opencode acp` from nixpkgs against an
OpenAI-compatible provider from acp.opencode.{provider,model,
contextWindow,outputLimit}: the config is rendered to the store with
the API key as an {env:VAR} reference, so the key is read at runtime
from backendEnvironmentFile. OPENCODE_PERMISSION denies opencode's
built-in bash, task, todowrite and websearch, and makes webfetch ask.
Refs #4391
1b24edf4b4