chore: post-#1442 cleanup — fix stale price example + drop dead CSS

Two small follow-ups after the modelPrices PR (#1442):
- nix/modules/hive-c0re.nix: the `modelPrices` option `example` still
  showed the old 5-minute sonnet `cache_write = 3.75`; the default is now
  the 1-hour TTL `6.0`. Bump the example to match so it doesn't mislead.
- frontend/packages/agent/agent.css: remove the dead `.stats-empty-note`
  selector (no element uses it — the stats empty state paints on canvas).

frontend/packages/agent/src/agent.css

@@ -1059,8 +1059,6 @@ pre.diff {
 .stats-card.wide { grid-column: 1 / -1; }
 .stats-card.wide .chart-wrap { height: 260px; }
 
-.stats-empty-note { color: var(--muted); font-style: italic; }
-
 /* ─── /screen page ─── */
 /* Full-screen VNC canvas viewer (screen.html). All rules scoped to
    body.screen-shell so they can coexist with the agent/stats rules. */

nix/modules/hive-c0re.nix

@@ -476,7 +476,7 @@ in
           input = 3.0;
           output = 15.0;
           cache_read = 0.3;
-          cache_write = 3.75;
+          cache_write = 6.0;
         };
       };
       description = ''