revert(test): keep opencode default on sonnet, not kimi

c45a07e5 swapped the opencode harness default to moonshotai/kimi-k2,
which broke push-enabled (Kimi reported delete_branch as "auth failed"
where sonnet handles it). The agnostic test matrix expects Sonnet-grade
tool-calling; switching to Kimi was scope creep on top of the original
"don't default to opus" ask — opencode's default was already sonnet, not
opus, so nothing needed to change there.

test/run.ts

@@ -312,7 +312,7 @@ async function runTestForAgent(ctx: RunContext): Promise<ValidationResult> {
     } else {
       const defaultModels: Record<string, string> = {
         claude: "anthropic/claude-sonnet-4-6",
-        opencode: "moonshotai/kimi-k2",
+        opencode: "anthropic/claude-sonnet-4-6",
       };
       const model = defaultModels[ctx.agent];
       if (model) {