refactor(agent): publish agent/inbox/claimed from Inbox.claim

Move the claimed-message notification loop out of the loop's pre-step
into Inbox.claim(target, turn), so the step-boundary operation publishes
its own claimed notifications like insertions and discards do.

packages/context/tmux-context/tests/tmux-context.spec.ts

@@ -96,7 +96,7 @@ function sessionAgent(session: Session, id = 'agent'): Agent {
     id: SessionId(id),
     options: {},
     session,
-    inbox: new Inbox(session, { inserted: () => {}, discarded: () => {} }),
+    inbox: new Inbox(session, { inserted: () => {}, discarded: () => {}, claimed: () => {} }),
     status: 'running',
     ctx: new Context(),
     send: () => {},