fix(agent-loop): balance inbox invariant on continuation-reason steer

The FIFO-conservation invariant fired on the loop-authored continuation
reason path: a continue-with-reason decision entered the steering FIFO
without an agent/inbox/enqueue, so its later dequeue/discard had no
matching enqueue. Emit the enqueue for that steer too, add a regression
test that mounts the invariant over a continue-with-reason turn and a
cancel, and hoist the duplicated inboxInfo helper into inbox.ts.

Found by fresh-eye review.

packages/core/agent-loop/src/inbox.ts

@@ -7,7 +7,7 @@
  */
 
 import type { ContentBlock, MessageSource } from '@deepseek-ai/dsh-llm'
-import type { HookContext } from '@deepseek-ai/dsh-agent'
+import type { HookContext, InboxItemInfo } from '@deepseek-ai/dsh-agent'
 
 /** One message waiting in an agent's inbox. */
 export interface InboxMessage {
@@ -18,6 +18,16 @@ export interface InboxMessage {
   wakeup: boolean
 }
 
+/**
+ * Build the `agent/inbox/*` event payload for one inbox item.
+ * @param message - the accepted inbox record.
+ * @param steering - whether the item is in the steering FIFO (`next-step`).
+ * @returns the live-event facts for enqueue/dequeue/discard.
+ */
+export function inboxInfo(message: InboxMessage, steering: boolean): InboxItemInfo {
+  return { content: message.content, source: message.source, contexts: message.contexts, steering, wakeup: message.wakeup }
+}
+
 /**
  * Per-agent inbox: a queued FIFO (dequeued once per turn start) and a steering FIFO
  * (drained between steps of a running turn). Purely an in-memory mechanism of