test: switch opus defaults to sonnet/kimi for cheaper coverage

- opencode test default: claude-sonnet-4-6 → moonshotai/kimi-k2. claude-code
  default stays on sonnet (the agent-under-test for that path is Claude
  Code); the opencode path doesn't need an Anthropic model.
- byok-no-keys-fallback fixture: anthropic/claude-opus → moonshotai/kimi-k2.
  test verifies fallback to opencode/big-pickle, so the configured model
  is never invoked; picking a non-opus alias avoids burning credits if
  the fallback ever regresses.
- BEDROCK_MODEL_ID in both test workflows: us.anthropic.claude-opus-4-6-v1
  → us.anthropic.claude-sonnet-4-6. env var is required by the ci.test
  invariant that every provider's env vars are wired into the workflow.

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: "anthropic/claude-sonnet-4-6",
+        opencode: "moonshotai/kimi-k2",
       };
       const model = defaultModels[ctx.agent];
       if (model) {