chore: set a 3,000-line custom-code budget (#2446)

## Summary

Set a 3,000-line ceiling on additions **plus** deletions in the
remaining custom patch against verified generated output. This is the
budget-only foundation for the same rollout approved in [Python
#3714](https://github.com/openai/openai-python/pull/3714) and [Python
#3715](https://github.com/openai/openai-python/pull/3715).

The verified current patch is **+1,589 / −181 = 1,770 lines**, leaving
**1,230 lines** of headroom. This is a generous bound on accidental
growth, not a target or permission to add custom code.

Only `.castiron-ratchet.json` changes here. The stacked tooling PR will
use the base-branch allowance, so a PR cannot raise its own budget.
Future increases require a separate budget-only PR, explicit
justification, and a human approving review; agents cannot approve them.

No workflow, CODEOWNERS, or ruleset changes are included. Ruleset
enforcement is a separate, later decision.

Tooling companion: https://github.com/openai/openai-node/pull/2447