feat: Add MiniMax Anthropic-compatible API support (minimax-api)

Add --auth-choice minimax-api for direct MiniMax API usage at
https://api.minimax.io/anthropic using the anthropic-messages API.

Changes:
- Add applyMinimaxApiConfig() function with provider/model config
- Add minimax-api to AuthChoice type and CLI options
- Add handler and non-interactive support
- Fix duplicate minimax entry in envMap
- Update live test to use anthropic-messages API
- Add 11 unit tests covering all edge cases
- Document configuration in gateway docs

Test results:
- 11/11 unit tests pass
- 1/1 live API test passes (verified with real API key)

Co-Authored-By: Claude <noreply@anthropic.com>

src/commands/onboard-auth.test.ts

@@ -7,6 +7,8 @@ import { afterEach, describe, expect, it } from "vitest";
 
 import {
   applyAuthProfileConfig,
+  applyMinimaxApiProviderConfig,
+  applyMinimaxApiConfig,
   writeOAuthCredentials,
 } from "./onboard-auth.js";
 
@@ -105,3 +107,113 @@ describe("applyAuthProfileConfig", () => {
     ]);
   });
 });
+
+describe("applyMinimaxApiConfig", () => {
+  it("adds minimax provider with correct settings", () => {
+    const cfg = applyMinimaxApiConfig({});
+    expect(cfg.models?.providers?.minimax).toMatchObject({
+      baseUrl: "https://api.minimax.io/anthropic",
+      api: "anthropic-messages",
+    });
+  });
+
+  it("sets correct primary model", () => {
+    const cfg = applyMinimaxApiConfig({}, "MiniMax-M2.1-lightning");
+    expect(cfg.agents?.defaults?.model?.primary).toBe("minimax/MiniMax-M2.1-lightning");
+  });
+
+  it("sets reasoning flag for MiniMax-M2 model", () => {
+    const cfg = applyMinimaxApiConfig({}, "MiniMax-M2");
+    expect(cfg.models?.providers?.minimax?.models[0]?.reasoning).toBe(true);
+  });
+
+  it("does not set reasoning for non-M2 models", () => {
+    const cfg = applyMinimaxApiConfig({}, "MiniMax-M2.1");
+    expect(cfg.models?.providers?.minimax?.models[0]?.reasoning).toBe(false);
+  });
+
+  it("preserves existing model fallbacks", () => {
+    const cfg = applyMinimaxApiConfig({
+      agents: {
+        defaults: {
+          model: { fallbacks: ["anthropic/claude-opus-4-5"] },
+        },
+      },
+    });
+    expect(cfg.agents?.defaults?.model?.fallbacks).toEqual(["anthropic/claude-opus-4-5"]);
+  });
+
+  it("adds model alias", () => {
+    const cfg = applyMinimaxApiConfig({}, "MiniMax-M2.1");
+    expect(cfg.agents?.defaults?.models?.["minimax/MiniMax-M2.1"]?.alias).toBe("Minimax");
+  });
+
+  it("preserves existing model params when adding alias", () => {
+    const cfg = applyMinimaxApiConfig({
+      agents: {
+        defaults: {
+          models: {
+            "minimax/MiniMax-M2.1": { alias: "MiniMax", params: { custom: "value" } },
+          },
+        },
+      },
+    }, "MiniMax-M2.1");
+    expect(cfg.agents?.defaults?.models?.["minimax/MiniMax-M2.1"]).toMatchObject({
+      alias: "Minimax",
+      params: { custom: "value" },
+    });
+  });
+
+  it("replaces existing minimax provider entirely", () => {
+    const cfg = applyMinimaxApiConfig({
+      models: {
+        providers: {
+          minimax: {
+            baseUrl: "https://old.example.com",
+            apiKey: "old-key",
+            api: "openai-completions",
+            models: [{ id: "old-model", name: "Old", reasoning: false, input: ["text"], cost: { input: 1, output: 2, cacheRead: 0, cacheWrite: 0 }, contextWindow: 1000, maxTokens: 100 }],
+          },
+        },
+      },
+    });
+    expect(cfg.models?.providers?.minimax?.baseUrl).toBe("https://api.minimax.io/anthropic");
+    expect(cfg.models?.providers?.minimax?.api).toBe("anthropic-messages");
+    expect(cfg.models?.providers?.minimax?.models[0]?.id).toBe("MiniMax-M2.1");
+  });
+
+  it("preserves other providers when adding minimax", () => {
+    const cfg = applyMinimaxApiConfig({
+      models: {
+        providers: {
+          anthropic: {
+            baseUrl: "https://api.anthropic.com",
+            apiKey: "anthropic-key",
+            api: "anthropic-messages",
+            models: [{ id: "claude-opus-4-5", name: "Claude Opus 4.5", reasoning: false, input: ["text"], cost: { input: 15, output: 75, cacheRead: 0, cacheWrite: 0 }, contextWindow: 200000, maxTokens: 8192 }],
+          },
+        },
+      },
+    });
+    expect(cfg.models?.providers?.anthropic).toBeDefined();
+    expect(cfg.models?.providers?.minimax).toBeDefined();
+  });
+
+  it("preserves existing models mode", () => {
+    const cfg = applyMinimaxApiConfig({
+      models: { mode: "replace", providers: {} },
+    });
+    expect(cfg.models?.mode).toBe("replace");
+  });
+});
+
+describe("applyMinimaxApiProviderConfig", () => {
+  it("does not overwrite existing primary model", () => {
+    const cfg = applyMinimaxApiProviderConfig({
+      agents: { defaults: { model: { primary: "anthropic/claude-opus-4-5" } } },
+    });
+    expect(cfg.agents?.defaults?.model?.primary).toBe(
+      "anthropic/claude-opus-4-5",
+    );
+  });
+});