Restore first-class OMO Slim council support (#1981) (#1982)

cc-switch could already persist arbitrary OMO Slim agent keys and top-level fields, but the built-in metadata and UI copy still reflected the pre-council agent set. This made the upstream council feature look unsupported and pushed users toward manual JSON-only setup.

Promote council to a built-in OMO Slim agent, add copy that points top-level plugin settings at Other Fields, and lock the behavior with regression tests.

Constraint: oh-my-opencode-slim exposes council through both agents.council and top-level council config
Rejected: Add a dedicated council editor UI now | too much surface area for issue #1981
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep OMO Slim built-in agent metadata aligned with upstream agent additions before shipping UI support
Tested: pnpm exec vitest run tests/utils/omoConfig.test.ts tests/components/OmoFormFields.mergeCustomModelsIntoStore.test.ts
Tested: pnpm typecheck
Not-tested: End-to-end validation against a live oh-my-opencode-slim installation
Related: farion1231/cc-switch#1981

tests/utils/omoConfig.test.ts

@@ -1,6 +1,9 @@
 import { describe, expect, it } from "vitest";
 import {
   buildOmoProfilePreview,
+  buildOmoSlimProfilePreview,
+  OMO_SLIM_BUILTIN_AGENTS,
+  OMO_SLIM_DISABLEABLE_AGENTS,
   parseOmoOtherFieldsObject,
 } from "@/types/omo";
 
@@ -28,3 +31,38 @@ describe("buildOmoProfilePreview", () => {
     expect(fromObject).toEqual({ foo: "bar" });
   });
 });
+
+describe("buildOmoSlimProfilePreview", () => {
+  it("保留 top-level council 配置，同时写入 council agent 模型", () => {
+    const preview = buildOmoSlimProfilePreview(
+      {
+        council: { model: "openai/gpt-5.4-mini" },
+      },
+      '{ "council": { "default_preset": "default" }, "fallback": { "enabled": true } }',
+    );
+
+    expect(preview).toEqual({
+      council: { default_preset: "default" },
+      fallback: { enabled: true },
+      agents: {
+        council: { model: "openai/gpt-5.4-mini" },
+      },
+    });
+  });
+});
+
+describe("OMO Slim metadata", () => {
+  it("将 council 视为内置且可禁用的 agent", () => {
+    expect(OMO_SLIM_BUILTIN_AGENTS).toContainEqual(
+      expect.objectContaining({
+        key: "council",
+        display: "Council",
+        group: "sub",
+      }),
+    );
+    expect(OMO_SLIM_DISABLEABLE_AGENTS).toContainEqual({
+      value: "council",
+      label: "Council",
+    });
+  });
+});