feat(opencode): add codex estimated cost flag

[major]

Issue for this PR

Closes#35133

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Keeps ChatGPT/Codex subscription auth showing actual marginal cost as $0.00 by default.

Adds OPENCODE_CODEX_SHOW_ESTIMATED_COST=true so users can opt in to API-equivalent estimated costs for Codex models.

Documents the flag and covers default/opt-in model pricing behavior.

How did you verify your code works?

• bunx prettier --check packages/opencode/src/effect/runtime-flags.ts packages/opencode/src/plugin/index.ts packages/opencode/src/plugin/openai/README.md packages/opencode/src/plugin/openai/codex.ts packages/opencode/test/plugin/codex.test.ts
• cd packages/opencode && bun test test/plugin/codex.test.ts
• cd packages/opencode && bun typecheck
• pre-push hook: bun turbo typecheck

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The only potentially related PR found is #30164 "feat(tui): show estimated live token throughput in footer", but this appears to be about displaying token throughput in the UI rather than adding a cost estimation flag for Codex. They are separate features.

No duplicate PRs found

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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