swarm-ui: agents page — explainer message when the roster is empty

Table gains an optional emptyMessage prop, rendered as a single
full-width row in place of a bare empty tbody; AgentsPage is the first
consumer. Screenshot-verified against a mock server returning an empty
roster.

frontend/packages/swarm-ui/src/pages/AgentsPage.tsx

@@ -115,7 +115,14 @@ export function AgentsPage() {
     >
       {error ? <ApiErrorPanel context="failed to load the agent roster" problem={error} /> : null}
       {!error && rows === null ? <p>loading…</p> : null}
-      {rows ? <Table columns={COLUMNS} rows={rows} rowKey={(a) => a.name} /> : null}
+      {rows ? (
+        <Table
+          columns={COLUMNS}
+          rows={rows}
+          rowKey={(a) => a.name}
+          emptyMessage="no agents yet — the swarm-wide identity store has no agents registered on any hive"
+        />
+      ) : null}
       <Dialog open={createOpen} onClose={() => setCreateOpen(false)} label="create agent">
         <CreateAgentForm />
       </Dialog>