fix(grok): correct opencode provider (Responses API), stdin, reasoning cost

A live opencode run against api.x.ai/v1 surfaced three real bugs:

1. Provider package — grok-build-0.1 is driven via the OpenAI Responses API
   (opencode calls model.responses()). @ai-sdk/openai-compatible is
   chat/completions only and errors "responses is not a function". Switch the
   generated opencode.json provider + the grok image to @ai-sdk/openai.
2. Headless hang — `opencode run` blocks after init without a TTY; close stdin
   (`< /dev/null`) in the entrypoint so it proceeds to the model call.
3. Reasoning-token cost — grok-build-0.1 is a reasoning model; reasoning tokens
   bill as output but opencode stores them in a separate column. cost_for_session
   folds tokens_reasoning into output (else ~22x undercount).

Verified end-to-end against a real session row (input=6120, output=1,
reasoning=226, cache_read=1856): our pricing reproduces opencode's stored USD
cost ($0.0069452) exactly. Tests anchored to that real row.

tests/unit/llm/test_opencode_config.py

@@ -71,7 +71,8 @@ def test_build_opencode_config_provider_and_model() -> None:
         instruction_paths=["/app/system-prompt.md"],
     )
     provider = cfg["provider"]["xai"]
-    assert provider["npm"] == "@ai-sdk/openai-compatible"
+    # grok-build-0.1 needs the Responses API → @ai-sdk/openai, not -compatible.
+    assert provider["npm"] == "@ai-sdk/openai"
     assert provider["options"]["baseURL"] == "https://api.x.ai/v1"
     assert provider["options"]["apiKey"] == "xai-key"
     assert "grok-build-0.1" in provider["models"]