Store the subagents and read them with zoom

Store the subagents and read them with zoom

Each subagent has a directory in subagents/ of the agent directory,
named for it, with its own message log. A name is an adjective and a
noun from two word lists, each capitalized and joined, such as
GreenOtter, and a new subagent draws again when the directory of its
name exists. Only such a name can be a directory, so a name never leaves
subagents/.

The zoom tool gets a nullable name, and zoom("Name") gives the whole log
of that subagent, each message as "i|kind: text". The prompt gets the
tool line for it. The log of a name opens once and stays open until the
daemon stops, because its lock allows one open log, and its mutex keeps
the run of the subagent and a zoom of the agent apart.

Issue: b1ac698