refactor(agent): runAgent uses Agent upstreams only; deprecate legacy settings.pi fields

pi.provider / baseUrl / model / thinkingLevel / apiKeySecretRef are now
DEPRECATED_SETTINGS_FIELDS: kept in the schema for stored JSON, dropped
from PATCH /settings and from the settings sections, never read by the
server. runAgent fails with a clear message when no upstream is set.
The agent demo gate checks the default upstream provider; e2e seeds a
fake upstream as default. OpenAPI summaries say Agent upstream.

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

apps/server/src/modules/pi/service.ts

@@ -205,7 +205,7 @@ export interface RunUpstream {
 }
 
 /**
- * 取运行用的上游:显式 upstreamId 优先,否则 settings.pi.defaultUpstreamId;都没有返回 null(调用方走旧字段兼容)。
+ * 取运行用的上游:显式 upstreamId 优先,否则 settings.pi.defaultUpstreamId;都没有返回 null(runAgent 直接失败,旧 settings.pi 字段已弃用)。
  * 上游停用 / 未登录 / 登录失效时抛出可读错误。
  */
 export async function resolveRunUpstream(ctx: ServerContext, upstreamId?: string | null): Promise<RunUpstream | null> {