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/agent/fake-scripts.ts

@@ -1,7 +1,7 @@
 import type { FakeScript, FakeStep, FakeToolResult } from "@umt/agent";
 import { z } from "zod";
 
-// fake 模式脚本(pi.provider = "fake"):测试直接注入 FakeScript;设置里的 agent.fakeScript 是可序列化子集(JSON):
+// fake 模式脚本(默认上游 provider = "fake"):测试直接注入 FakeScript;设置里的 agent.fakeScript 是可序列化子集(JSON):
 //   { "match": "收进来" | { "regex": "..." }, "steps": [{ "text"?, "toolCalls": [{ "capability", "input" }] }], "finalText": "..." }
 // 或它们的数组。input 里的字符串可以写 "{{download.plan.plan.id}}":取该能力最近一次成功结果里的路径值。