IBuildNotifier that shouts on the host's hivemind.

The host is a runtime, not a dependency: its shout function is resolved
when a notice is delivered, never required. Outside a host the notifier
answers with an err Result and the watcher carries on.

A shout reaches the model two ways. It is piggybacked on the next tool
result of the reader it is addressed to, and sixth-sense perceives it as
`:sense/build-failed` (see hive-elm.notify.sense) and pushes it to an idle
session. Both address by `:parent-id`: without one the shout goes to
coordinator readers only.

Registers `:sense/build-failed` with sixth-sense, when it is loaded.

sixth-sense turns a shout into a sense only when a registered class
perceives it; a `:build-failed` shout matches none of the built-ins. The
registration is one map. sixth-sense belongs to hive-agent, which this
library does not depend on, so its registry is resolved softly: without
it the shout is still piggybacked, only never pushed.