Bound review/adversarial diff context size #32

Summary

Review prompt construction currently embeds the full local diff or PR diff into the prompt without a total size cap. Large local diffs or PRs can exceed provider input limits, make reviews unreliable, or waste tokens.

The upstream Codex issue included spawnSync ENOBUFS; this repo avoids that exact sync-buffer failure by using async spawn, but the provider input-size problem still applies.

Local evidence

plugins/opencode/scripts/lib/prompts.mjs:

• buildReviewPrompt() calls getDiff(...) or getPrDiff(...).
• buildReviewContext() emits the entire diff as:

<diff>
${diff}
</diff>

There is no per-file cap, total cap, binary patch guard, or fallback path.

Expected behavior

Large reviews should fail gracefully or degrade into a bounded review context instead of blindly sending unlimited diff text.

Suggested fix

Add review-context budget handling:

1. Enforce a configurable total character/token budget for <diff>.
2. Enforce per-file caps and skip obvious binary/generated blobs.
3. Include changed-file list and stats even when truncating the diff.
4. Mark truncation clearly in the prompt so OpenCode knows the review is partial.
5. For very large reviews, consider a separate mode that asks OpenCode to inspect the repo/diff itself with bounded file reads instead of embedding the entire diff.

Upstream references

Derived from openai/codex-plugin-cc#11 and #69. Related local feature direction: #21 for path/folder review context with size caps.