[FEATURE] Add a setting to suppress plan usage-limit warnings (5-hour / weekly)

What I want

An opt-out for the plan usage-limit warnings — "You've used 80% of your 5-hour limit · resets 3pm", "Approaching weekly limit" — as a settings.json key (e.g. "hideUsageLimitWarnings": true) or an env var alongside the existing DISABLE_COST_WARNINGS.

Why

I already render live usage from the rate-limit headers in my custom status line, so the banner is duplicate information that interrupts the transcript at exactly the moments I'm most focused. Anyone running a usage-aware status line is in the same position.

Current state (checked against 2.1.226)

There is no way to turn these off client-side:

• DISABLE_COST_WARNINGS gates cost/spend warnings only — it does not gate the plan rate-limit banners.
• hideRateLimitsDescription exists on the plan-config object, but it is server-delivered as part of the plan/org profile, so a user cannot set it.
• The warning is emitted whenever anthropic-ratelimit-unified-status: allowed_warning comes back. The only client-side suppressions are utilization < 0.7 and the team/enterprise-with-extra-usage case; no user preference is consulted.
• There is no /config toggle, and no settings-schema key matching *Warning* for usage.

Notes

Hard rejections (status: rejected, "You've hit your limit") should keep showing regardless — this is only about silencing the pre-emptive allowed_warning tier.

Related: #31918 (configurable warning threshold) was closed as stale rather than declined, and #20636 exposed usage percentages to statusLine, which is exactly what makes the banner redundant for this setup.