_TasksLifecycleBasePromote nested named agents into typed handles on run.subagents.
The base _TasksLifecycleBase records each namespace's lc_agent_name
(set by create_agent(name=...)) and, on every task start, fires
_on_started with the resolved graph_name and a cause for genuine
subagent boundaries. This transformer gates on that boundary using the
inherited _lc_by_ns map: a nested run is a subagent when it carries an
lc_agent_name. Same-named nested agents (e.g. a subagent that invokes
itself) are surfaced; unnamed agents (None) are excluded. Trade-off: a
non-agent subgraph that inherited the parent's name will also surface.
On the first matching task start it builds a child mux and emits a typed
handle on run.subagents, then forwards subsequent child-scope events into
that handle so the nested run can be consumed independently.