OpenCode review tool fails with long background context

### OpenCodeReview Version

```text
open-code-review v1.9.7 (f269d0ce0) darwin/arm64
```

### Operating System

macOS (Apple Silicon)

### Installation Method

npm (global)

### LLM Provider

Other OpenAI-compatible endpoint

### Bug Description

The official OpenCode plugin exposes only a `background` string and always passes it to OCR through `--background`.

With multi-paragraph review context:

- `ocr_review` preview succeeds.
- The full `ocr_review` call exits immediately without creating an OCR session.
- OpenCode reports only `OpenCodeReview exited with code 1.`
- Running the same review and context directly with `--background-file` creates the OCR session and proceeds normally.

The adapter also discards the child process termination signal. Its `close` handler converts a null exit code to `1`, so the caller cannot distinguish a signal termination from a normal exit and receives no useful diagnostic when stdout and stderr are empty.

### Steps to Reproduce

1. Install the official OpenCode plugin from this repository.
2. Invoke `ocr_review` in preview mode for a branch range and confirm the file selection succeeds.
3. Invoke the full review with a multi-paragraph `background` value.
4. Observe the generic exit-code error and confirm that `ocr session list` contains no new session.
5. Save the same background text to a Markdown file and run:

```shell
ocr review --from main --to feature --background-file context.md
```

6. Observe that OCR creates a session and starts reviewing normally.

### Expected Behavior

The OpenCode adapter should support large review context without silently losing the OCR process.

Suggested options:

- Expose a `backgroundFile` tool argument.
- Transparently write long background text to a private temporary file and pass `--background-file`.
- Remove the temporary file on success, failure, or cancellation.
- Preserve and report the child process exit signal alongside stdout and stderr.

### Logs / Error Output

```shell
OpenCodeReview exited with code 1.
```

No OCR session is created for the failed tool call and no stdout or stderr is returned.

### Additional Context

The failure was reproduced twice through the OpenCode tool. LLM connectivity and OCR preview both passed immediately before the full calls. The direct `--background-file` run used the same provider, refs, and review context.

OpenCode version: `1.18.18`.

Start at the OpenCode adapter handling the ocr_review tool and its child-process close handler. Compare the existing --background path with the working --background-file invocation, then reproduce the multi-paragraph case. Done means large context completes without losing the OCR session and failures report the termination signal plus available output.

Written by the indexing model from the issue text.