stats: per-agent /stats page with chart.js trends + breakdowns

new hive-ag3nt::stats module reads turn_stats.sqlite read-only and
aggregates over 24h/7d/30d windows (hourly/daily buckets) — turn
rate, p50/p95/avg duration, ctx tokens (avg/max), cost token
components, top tools, wake mix, result mix. served by the agent
itself so per-MCP extensions can register more providers without
the host knowing their schemas.

/stats route + /api/stats?window=... on the per-agent web ui.
chart.js v4.4.4 pulled from jsdelivr (SRI hash deferred). nav
links: 📊 chip on the dashboard container row + 📊 stats → on
the per-agent header.

todo housekeeping: softened damocles-area note at the top,
new reverse-proxy + deferred reminder-rollup items, removed
the two telemetry-ui items absorbed by this page.

hive-c0re/assets/app.js

@@ -487,6 +487,13 @@
         el('a', { class: 'name', href: url, target: '_blank', rel: 'noopener' }, c.name),
         el('span', { class: c.is_manager ? 'role role-m1nd' : 'role role-ag3nt' },
           c.is_manager ? 'm1nd' : 'ag3nt'),
+        el('a', {
+          class: 'meta',
+          href: url + 'stats',
+          target: '_blank',
+          rel: 'noopener',
+          title: 'per-agent stats page (turn rate, durations, tokens, tool mix)',
+        }, '📊'),
       );
       if (pending) {
         head.append(el('span', { class: 'pending-state' },