fix(minimax): use correct API endpoint and format

MiniMax has updated their API. The previous configuration used an
incorrect endpoint (api.minimax.io/anthropic) with anthropic-messages
format, which no longer works.

Changes:
- Update MINIMAX_API_BASE_URL to https://api.minimax.chat/v1
- Change API format from anthropic-messages to openai-completions
- Remove minimax from isAnthropicApi check in transcript-policy

This fixes the issue where MiniMax API calls return no results.

src/agents/transcript-policy.ts

@@ -51,7 +51,8 @@ function isOpenAiProvider(provider?: string | null): boolean {
 function isAnthropicApi(modelApi?: string | null, provider?: string | null): boolean {
   if (modelApi === "anthropic-messages") return true;
   const normalized = normalizeProviderId(provider ?? "");
-  return normalized === "anthropic" || normalized === "minimax";
+  // MiniMax now uses openai-completions API, not anthropic-messages
+  return normalized === "anthropic";
 }
 
 function isMistralModel(params: { provider?: string | null; modelId?: string | null }): boolean {