feat(opencode-go): route by request format via transports + per-model guard

opencode-go hard-coded targetFormat: claude per model, so every client
format was force-routed to /messages (Codex/OpenAI clients paid a lossy
Responses->OpenAI->Claude double translation). Declare the existing
upstream multi-endpoint transports [openai, claude, openai-responses]
and guard per model via registry supportedFormats: kimi/glm/mimo only
support /chat/completions, minimax/qwen add /messages, deepseek adds
/responses. Undeclared models keep the upstream default.

Drop the bespoke OpenCodeGoExecutor (its shared _lastModel cache could
cross auth headers between concurrent requests); DefaultExecutor already
consumes runtimeTransport and injects reasoning content.

open-sse/executors/opencode-go.js

@@ -1,49 +0,0 @@
-import { BaseExecutor } from "./base.js";
-import { PROVIDERS } from "../config/providers.js";
-import { injectReasoningContent } from "../utils/reasoningContentInjector.js";
-import { ANTHROPIC_API_VERSION } from "../providers/shared.js";
-
-// Models that use /zen/go/v1/messages (Anthropic/Claude format + x-api-key auth)
-const MESSAGES_FORMAT_MODELS = new Set([
-  "minimax-m3",
-  "minimax-m2.7",
-  "minimax-m2.5",
-  "qwen3.7-max",
-  "qwen3.7-plus",
-  "qwen3.6-plus",
-]);
-
-const BASE = "https://opencode.ai/zen/go/v1";
-
-export class OpenCodeGoExecutor extends BaseExecutor {
-  constructor() {
-    super("opencode-go", PROVIDERS["opencode-go"]);
-  }
-
-  // buildUrl runs before buildHeaders in BaseExecutor.execute, cache model here
-  buildUrl(model) {
-    this._lastModel = model;
-    return MESSAGES_FORMAT_MODELS.has(model)
-      ? `${BASE}/messages`
-      : `${BASE}/chat/completions`;
-  }
-
-  buildHeaders(credentials, stream = true) {
-    const key = credentials?.apiKey || credentials?.accessToken;
-    const headers = { "Content-Type": "application/json" };
-
-    if (MESSAGES_FORMAT_MODELS.has(this._lastModel)) {
-      headers["x-api-key"] = key;
-      headers["anthropic-version"] = ANTHROPIC_API_VERSION;
-    } else {
-      headers["Authorization"] = `Bearer ${key}`;
-    }
-
-    if (stream) headers["Accept"] = "text/event-stream";
-    return headers;
-  }
-
-  transformRequest(model, body) {
-    return injectReasoningContent({ provider: this.provider, model, body });
-  }
-}