codex-executor: bake in a tight default review prompt + output-format contract

Problem

codex-executor.yml forwards the consumer's prompt: to the model verbatim (only appending a guardrail + "Be concise.") and applies no output-format contract. So output length is entirely at the mercy of each consumer's prompt.

Concretely: dotCMS/Infrastructure-as-code's auto-review prompt enumerated 9 review dimensions and asked the model to "be constructive and helpful" → gpt-5.4 produced a 243-line essay for a 7-line diff (see IaC #7889). Tightening that repo's prompt fixed it, but every other consumer can still trip the same wire.

By contrast, bedrock-generic-executor.yml already ships a sensible tight default prompt:

"Review this PR diff. Flag anything that looks wrong, risky, or worth a second look… Skip praise. If it is clean, say so in one line."

codex-executor has no equivalent.

Ask

1. Give codex-executor a tight default prompt (parity with bedrock-generic-executor) for when the consumer omits prompt:.
2. Add a light output-format contract in the executor's instructions so consumer prompts can't blow up length:
   • one line per finding, severity + file:line — why
   • no praise / summaries / style nitpicks
   • "if clean, say so in one sentence"
   • "prefer returning nothing over manufacturing low-value findings"

This makes good output the default instead of something each repo has to re-derive in its own prompt.

Notes / gotcha for whoever picks this up

• gpt-5.x half-echoes a literal **[…]** severity template (emits a doubled **[**[). Use a bracketless tag in any example — 🟠 **High**, not **[🟠 High]**. Verified on IaC #7889.

Refs: IaC #7889, core #36182 (tight prompt lives in core .github/prompts/gpt-auto-review.md).

Start with codex-executor.yml and compare its prompt handling with the default in bedrock-generic-executor.yml. Update the codex executor's default prompt and instructions to define concise finding output, clean-result behavior, and bracketless severity examples. Done means omitted prompts receive the tight default and consumer prompts have the stated output contract.