mara on #778: 'remove the manager special case argus nitted about'.
`plugins::install_configured` no longer takes a `notify_recipient`
hardcoding "manager". Now returns a Vec<String> of failure messages;
serve_main<S> iterates them and routes each through S::send_to_parent
— the same <parent> sentinel failure-notify uses everywhere else
(#703). Manager plugin failures now reach operator via root → operator
fallback (improvement on the pre-PR silent-drop).
Also rename FORGE_MENTIONS_ONLY → FORGE_IS_MANAGER to fix the misnomer:
the boolean picks which wire enum (AgentRequest::Wake vs
ManagerRequest::Wake) the forge_notify poller uses, not anything about
mentions-only filtering (that's a separate nix-side option). Real fix
is to lift Surface into the lib crate and make forge_notify::run
generic; deferred to its own issue.
Net: -20 LOC.
df71d8deac