agent page: inbox view of last 30 messages addressed to this agent

new wire request AgentRequest::Recent { limit } / ManagerRequest::Recent
(plus matching responses with Vec<InboxRow>). InboxRow moved to
hive-sh4re so it lives on both surfaces without an internal-to-wire
conversion. host-side dispatch in agent_server / manager_server
calls broker.recent_for(name, limit).

per-agent web_ui /api/state grew an inbox: Vec<InboxRow> populated
via the same per-agent socket (best-effort; transport failure
returns empty). frontend renders as a collapsible <details> section
between the state row and the terminal — fmt timestamp / from /
body in a tight grid, capped at 16em scrollable. only visible when
there are rows.

hive-c0re/src/manager_server.rs

@@ -100,6 +100,12 @@ async fn dispatch(req: &ManagerRequest, coord: &Coordinator) -> ManagerResponse
                 message: format!("{e:#}"),
             },
         },
+        ManagerRequest::Recent { limit } => match coord.broker.recent_for(MANAGER_AGENT, *limit) {
+            Ok(rows) => ManagerResponse::Recent { rows },
+            Err(e) => ManagerResponse::Err {
+                message: format!("{e:#}"),
+            },
+        },
         ManagerRequest::Recv => match coord
             .broker
             .recv_blocking(MANAGER_AGENT, MANAGER_RECV_LONG_POLL)