docs: Describe the compact OpenAI review report

[idy]

Background

The OpenAI review workflow now publishes one compact report with a single conclusion, three stage verdicts, a short scope and usage line, and folded technical details. The README still describes the previous report heading and does not explain which information remains visible by default.

Goal

Document the compact OpenAI PR review report so maintainers can distinguish the visible decision summary from the folded metadata and per-stage token/cache details.

This issue owns only the README description. It does not change workflow behavior, review policy, check names, cache behavior, or credit calculation.

Code Changes Tree

README.md    # describe the compact report layout and folded details

Design

Update the OpenAI review capability description to state that:

• one native OpenAI PR Review report contains the combined conclusion;
• PR format, Issue design, and code/plan conformance remain visible as three stage verdicts;
• scope and aggregate usage remain visible in the report summary;
• session metadata and per-stage token/cache details remain available in folded sections.

Keep the existing explanation of three native Checks and token-derived credit estimation. Do not duplicate the workflow implementation or add screenshots whose output may become stale.

Test And Acceptance Criteria

• README.md accurately describes the report generated by .github/workflows/codex-openai-review.yml.
• The documentation names OpenAI PR Review, OpenAI Issue Review, and OpenAI Code Review.
• The change is limited to README.md.
• git diff --check passes.