Make rate limit warning threshold configurable

Feature Request

The CLI currently displays a yellow rate limit warning when usage hits 90% of the current rate limit window. This threshold is hardcoded and not configurable.

Request: Allow users to configure the percentage threshold at which the rate limit warning appears, e.g. via an environment variable like CLAUDE_CODE_RATE_LIMIT_WARN_PCT.

Use case: Some users want earlier warnings (e.g. 70-80%) to adjust their workflow before hitting the limit, while others may prefer a later warning or no warning at all. DISABLE_COST_WARNINGS=1 exists as an all-or-nothing toggle, but there's no middle ground.

Proposed solution: An environment variable (e.g. CLAUDE_CODE_RATE_LIMIT_WARN_PCT=80) that accepts a percentage value. Default remains 90% for backward compatibility.