feat(server): gate scheduling, search and login by site capabilities (IA-2)

- snapshots only for sites with list; feed polling only for RSS kinds;
  user data / health checks only for sites with login; search site set
  only includes searchable sites; live search / snapshot skip with
  reason unsupported instead of hitting the site
- refresh-login / credentials need login, manual snapshot needs list (400)
- POST /sites accepts kind and rssUrl (site + feed in one transaction,
  key generated from host); RSS belongs to a site: feeds created without
  siteId go to the same-host custom RSS source site, siteId cannot be
  cleared
- fix: secret name clash when a site gets a second default-named feed
- command palette regrouped; agent site.list returns kind and
  capabilities; system prompt notes RSS lives under sites

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/capability.ts

@@ -357,7 +357,7 @@ export const CAPABILITY_CATALOG: Record<KnownCapabilityId, CapabilityDescriptor>
   "site.list": d(
     "site.list",
     "列出站点",
-    "列出已登记站点的健康、登录状态与账号摘要（不含凭证）。排查站点问题或确认站点可用时用",
+    "列出已登记站点的类型（PT 站 / 公开 BT 站 / 自定义 RSS 源）、可用的获取方式、健康、登录状态与账号摘要（不含凭证）。RSS 属于站点：自定义 RSS 源也是一种站点。排查站点或 RSS 问题、确认站点能不能搜索时用",
     "read",
     "SiteList",
     "read",