Why: complete agents delegate; the kernel must not nest executions
itself (orchestration stays with the host per the agent-execution
standard), so the missing piece is a port plus a tool face.
Evidence: go vet ./... clean; go test ./... 14 packages ok. Cases:
nil factory registers nothing, full delegation field round-trip
(goal/inputs/write_scope/non_goals/done_criteria/deliverables) with
sorted file list in the result text, sub-agent failure and factory
error propagation, schema-required goal rejection, pre-canceled
delegations never starting, and adapter end-to-end wiring through
Options.Subagent.
Impact: new runtime/subagent package (Request/Result/Factory); the
delegation message set matches the agent-execution delegation clause
(sub-tasks derive no extra permissions; budget fields are host-set);
native.Options gains Subagent.
Refs: plan B8; standards/agent-execution.md delegation section.
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