Send messages to subagents with tell

The agent gets the tell tool: tell(name, message) logs the message as a
user message of the subagent. A running subagent reads it between its
tool calls, and when it comes after a final answer, the run goes on with
one more request chain that reads the log again. The report of the run
then holds the final answer of each chain. A finished subagent goes on
from its log with a new run, with the view of now and the effort of the
note at the start of its log, and its reply comes back as a new report.

The log holds every message of a tell, so a told subagent needs no state
outside its log, and a restart can resume it in the same way.

Issue: b1ac698