The agent-default-model refactor moved the headless runner off the
api-proxy create path onto a direct agents.create, which silently dropped
the preset composition the stack had put on every dsh run session: the
header lost its agentPreset and the agent ran on the host composition
alone. Resolve the default preset up front (optional service — a roster-
less composition keeps the pre-preset behavior), record it on the header,
and mount it in setup where a broken composition still fails the run.