feat(core): make turn cancellation explicit

packages/core/agent/src/dispatch.ts

@@ -115,8 +115,9 @@ export function agentEvents(ctx: Context, agent: Agent): AgentEventDispatch {
  * Build the prompt assembly context with agent and scope set together, so
  * agent-scoped prompt and tool contributions cannot be silently omitted.
  * @param agent - the agent the assembly is for.
+ * @param signal - the current turn's explicit control signal, when assembly belongs to a turn.
  * @returns the context to pass to `assemble()`.
  */
-export function assembleContextFor(agent: Agent): AssembleContext {
-  return { agent, scope: agent }
+export function assembleContextFor(agent: Agent, signal?: AbortSignal): AssembleContext {
+  return { agent, scope: agent, ...signal === undefined ? {} : { signal } }
 }