swarm-controller: send one operator message to several agents

The swarm UI's broadcast page (#5524) needs the controller to do the
fan-out, so POST /api/agents/broadcast takes {agents, body} and answers
200 with {results: [{agent, sent, reason?}]} in request order.

- agent_input.rs: the single-agent send is now send_one, which both
  routes call; the single route maps its outcome to the same statuses
  and texts as before. The broadcast validates the whole request first
  (sender, empty body, empty or duplicated agents, more than 64, a name
  outside the roster), then sends up to 16 at once with the single
  route's no-responders window.
- main.rs: wires the route.
- docs/swarm/ui.md: describes the route next to the single-agent one.

swarm-controller/src/main.rs

@@ -3697,6 +3697,7 @@ fn build_app(state: AppState) -> axum::Router {
         .routes(routes!(subagent_term::stream_subagent_term))
         .routes(routes!(agent_state_stream::stream_agent_state))
         .routes(routes!(agent_input::post_agent_input))
+        .routes(routes!(agent_input::post_agents_broadcast))
         .routes(routes!(agent_control::post_agent_cancel))
         .routes(routes!(
             claude_login::post_claude_login,