fix(agent): avoid duplicate Codex custom prompt

[buger]

Summary

• keep Probe-authored Codex native system prompts as the single copy of custom role instructions
• preserve direct Codex engine helper behavior for callers that pass separate system/custom prompts
• prove preview/runtime prompt parity with a local zero-network fake Codex process

Root cause and evidence

The retained EXP-0210 parser-core/spec_review preview was 219,822 bytes against the closed 131,072-byte governed prompt limit because Probe forwarded the already-embedded custom prompt a second time. The zero-model localizer made one preview, zero initialize/answer/model/network calls, and preserved the checkpoint.

Verification

• node --test tests/unit/governed-answer-dispatch-preview.test.js tests/unit/governed-codex-runtime-binding.exp-0148.test.js (57 passed)
• git diff --check
• independent Sol/xhigh review: ACCEPT

No prompt limits, Proof authority, Visor bindings, schema validation, retry, or fallback behavior are weakened.