swarm: cancel an agent's turn from the swarm queue

A separate $SWARM.control.<agent> subject carries commands, apart from
the text subject. swarm-controller exposes POST /api/agents/{name}/cancel
as a NATS request; the harness answers whether a turn was actually
stopped, through the same cancel path as the dashboard button. The agent
credential gets subscribe on its control subject and allow_responses to
answer; only the controller may publish there.

Refs #3803

swarm-controller/src/main.rs

@@ -41,6 +41,7 @@ use swarm_authelia_bridge_sock::BridgeResponse;
 use utoipa::{OpenApi, ToSchema};
 use utoipa_axum::{router::OpenApiRouter, routes};
 
+mod agent_control;
 mod agent_icon;
 mod agent_identity;
 mod agent_input;
@@ -3424,6 +3425,7 @@ 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_control::post_agent_cancel))
         .routes(routes!(schedules::list_schedules, schedules::add_schedule))
         .routes(routes!(
             schedules::edit_schedule,