Auto-switch to fast mode when hitting standard mode usage limits

Feature Request

Current behavior: When hitting the standard (non-fast) Opus usage limit, Claude Code stops until the limit resets. Fast mode already has automatic fallback to standard mode built in, but not the reverse.

Desired behavior: When hitting the standard mode usage limit, automatically switch to fast mode (same model, higher throughput) until the limit resets, then switch back. This would mirror the existing fast→standard fallback that already works.

Why: Users are charged regardless of which mode they're in, so sitting idle during a standard mode cooldown is wasted value. Since fast mode uses the same Opus 4.6 model with no quality trade-off, automatically switching to it during cooldown periods would keep work flowing while the user is already paying for access. The bidirectional fallback would maximize productivity regardless of which mode hits a limit first.

Current workaround: Run in fast mode by default and rely on the existing fast→standard fallback. But this doesn't help users who prefer standard mode as their default.