Replaces the broadcast button in the messages card's header: a Disclosure
titled broadcast, folded by default, over the messages card. The composer is
built on the first open and kept, so it polls the agent list only once used.
The broadcast page wording in broadcast-types.ts and its test now says
composer.
The messages page gets a broadcast button that opens the agent picker and
message box above the stream, so a sent message shows up in the list below
it. The broadcast route and nav tab go, with no redirect: the page is now the
composer, BroadcastComposer, in the messages page.
Each polling page repeated its own fetch-and-set code on a fixed timer,
so a slow answer could be overtaken by the next tick, an older response
could overwrite a newer one, and nothing aborted a request when the page
went away.
usePolled skips a tick while a request is in flight, aborts on unmount
or refresh, ignores answers from dropped requests, and keeps the last
good data under an error. The alerts, hives, agents, schedules, builds,
review runs and broadcast pages and the subagent list use it.
Pick agents from a grid of their icons (running or all), type the text, send.
The page posts the names and the text to the controller and shows what
happened to each agent, with a retry that selects just the ones that failed.
The selection rules and the controller's answer are read by plain functions
with tests.
