[Feature] Team plan: ask the member to confirm before switching into extra-usage (usage credits)

Problem

On the Team plan, once an Owner enables usage credits, a member who reaches
their included quota starts consuming paid extra usage automatically, with
no per-member confirmation. Members can unintentionally spend without an
explicit, in-the-moment decision, and individual members have no self-service
control — only org Owners do.

Requested behavior

When a Team member is about to cross from included quota into extra usage,
Claude Code should pause and ask that member to confirm ("Included quota
reached — switch to extra usage? y/n") instead of silently auto-consuming.

Concrete proposal: extend the existing Pro/Max /usage-credits flow to Team seats

On Pro/Max today, a member can run /usage-credits to set their own monthly cap
and gets an in-CLI confirmation prompt when the limit is reached. Team seat
members don't have this — extra usage is Owner-gated and auto-consumed. The
mechanism already ships; it just isn't exposed to Team seats.

1. Per-seat self-service. Let a Team seat member run /usage-credits for

their own seat: set a personal monthly cap and toggle a per-member
"extra-usage" opt-in (default OFF).

1. Confirm before crossing. When opt-in is OFF and the member reaches the

included quota, pause and prompt instead of silently auto-consuming.

1. Owner guardrails stay authoritative. A member's personal cap can only be

≤ the Owner-set per-seat / org spend limit, and Owners can force opt-in
OFF org-wide. No member can exceed org policy.

1. Visibility. Surface remaining included vs. extra usage in /usage for

Team seats (ties into #48222).

Why this is worth revisiting (vs. #47111)

#47111 raised the same core problem and was closed as not planned, but as an
abstract "add a prompt" request. This narrows it to a concrete, low-surface
implementation: expose the already-shipping Pro/Max /usage-credits opt-in +
confirmation to Team seats, bounded by existing Owner spend limits. It is
additive (no new spending surface), opt-in by default, and directly resolves the
gap #47111 hit ("only org owners can set controls") without per-request Owner
round-trips.

Related: #47111 (closed as not planned), #48222 (extra-usage visibility on Team).