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/agent_server.rs

@@ -121,5 +121,11 @@ async fn dispatch(req: &AgentRequest, agent: &str, broker: &Broker) -> AgentResp
                 message: format!("{e:#}"),
             },
         },
+        AgentRequest::Recent { limit } => match broker.recent_for(agent, *limit) {
+            Ok(rows) => AgentResponse::Recent { rows },
+            Err(e) => AgentResponse::Err {
+                message: format!("{e:#}"),
+            },
+        },
     }
 }