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/providers/models/schema.js

@@ -38,3 +38,11 @@ export function modelStrip(model) {
 export function modelTargetFormat(model) {
   return model?.targetFormat || MODEL_DEFAULTS.targetFormat;
 }
+
+// Per-model declared upstream formats (e.g. ["openai", "claude"]). Guards the
+// sourceFormat-matched transport for multi-endpoint providers whose models differ
+// in endpoint support (opencode-go: kimi/glm only do /chat/completions, minimax/qwen
+// also do /messages, deepseek also does /responses).
+export function modelSupportedFormats(model) {
+  return model?.supportedFormats || null;
+}