feat: OpenCode Go usage monitor — hybrid collector + 246-centric dashboard

- usage_collector.py v3.0: HTTP+stored cookies (primary) + CDP (bonus)
  - Cookie keepalive: captures Set-Cookie from responses, saves back to JSON
  - Per-metric regex for SSR parsing (handles nested \[N]={...} structure)
  - Cache merge: preserves other accounts' data on partial collection
- extract_cookies.py: one-time CDP /cookies endpoint → per-account JSON
- dashboard.py: /api/usage reads local file (no Windows bot dependency)
  - /api/usage/refresh: local subprocess collector
  - Auto-timer: 5min background thread (cookie keepalive + fresh data)
- xmpp_agent_core.py: /usage endpoint + auto-timer (for local testing)
- dashboard.html: usage-monitor section (4 cards × 3 metrics + alerts)
- accounts.json: 4 accounts configured (key1-4, gitignored)
- .gitignore: cookies/, accounts.json, usage_stats.json

.gitignore

@@ -21,3 +21,8 @@ mowechat.conf
 .DS_Store
 Thumbs.db
 .venv/
+# OpenCode Go Usage Monitor
+gateway/scripts/usage_monitor/cookies/
+gateway/scripts/usage_monitor/accounts.json
+gateway/temp/usage_stats.json
+gateway/logs/usage_collector.log