Rename OSS_PROXY_MODEL to DEFAULT_PROXY_MODEL; default Router proxy to Kimi K2.6.

Derive the platform default from moonshotai/kimi-k2 openRouterResolve, add models.dev drift coverage, and promote repos on default-branch workflow pushes when still needs_setup.

external.ts

@@ -29,6 +29,7 @@ export function formatMcpToolRef(agentId: AgentId, toolName: string): string {
 // model alias registry lives in models.ts — re-exported here for shared access
 export type { ModelAlias, ModelProvider, ProviderConfig } from "./models.ts";
 export {
+  DEFAULT_PROXY_MODEL,
   getModelEnvVars,
   getModelManagedCredentials,
   getModelProvider,