Explain what this is and how to run it

Written for somebody who has just cloned it: the two commands that start it,
the mention that addresses the agent, four things worth trying, and the
configuration that matters.

Three notes are there because they are the traps. The dialect is an allowlist
and rejects anything it does not describe. The permission gate is the only
boundary, since there is no sandbox. And a node type the serialiser cannot
write throws inside the first update listener, which stops every listener after
it — collaboration included — without saying anything.

Verified by cloning into a fresh directory and following it exactly: install
resolves the pinned native CLI, types and 224 tests pass, the documented start
command boots with an agent, the documented URL answers, and Ctrl-C leaves
nothing behind.