New group_pool overage scope for teams sharing a rate-limit budget

What

• A new overageScope/limitScope value, group_pool, is now recognized alongside the existing org spend-cap and channel scopes, representing a team's shared usage budget rather than an individual member's own cap.
• Rate-limit messaging now has dedicated copy for this case: "You've hit your team's shared budget. Switch to another model... to continue," and when overage purchasing is available, "Run /usage-credits to raise it and keep using ${model} or switch models..." (otherwise it points to /model to switch models). It also points admins/users at claude.ai/admin-settings/usage.
• The Claude-in-Slack usage schema (CLAUDE_IN_SLACK_V2) gained an optional limitScope field with values service/channel/group_pool, so denied requests in Slack report which kind of limit was hit.
• The scope value itself is now parsed generically from a response header via a helper, instead of being hardcoded to the literal string 'channel', so other scope values (like group_pool) can be recognized and propagated through telemetry.

Why

This lets Claude Code tell users precisely when they've hit a team-wide pooled budget rather than their own individual limit, and point them at the right next step (raising the pool, switching models, or contacting an admin) instead of a generic rate-limit message.