[Feature] Let Team seat members self-manage extra usage via /usage-credits (confirm before auto-consuming)

Summary

Let Team seat members self-manage their own extra usage via the existing
Pro/Max /usage-credits flow, and confirm before crossing from included
quota into paid extra usage — instead of today's silent, Owner-gated
auto-consumption.

Please don't auto-close as a duplicate

This is filed as a concrete, implementation-focused follow-up — not a restated
duplicate:

• #47111 raised the same underlying problem and was closed as stale with the

explicit note: "Please open a new issue if this is still relevant."

• A prior attempt (#68346) was then flagged by the duplicate bot for auto-closure

against that very closed issue — a catch-22 (told to open a new one, then
closed for being a "duplicate" of the closed one).

• #47111 is closed and cannot serve as the canonical tracker, so this issue is

the live, concrete version.

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. Individual members have no self-service control — only
org Owners do — so credits can burn silently on routine work.

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. 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 ("Included quota reached — switch to extra
usage? y/n") 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 doing

Additive (no new spending surface), opt-in by default, bounded by existing Owner
controls, and it reuses the Pro/Max /usage-credits UX. It resolves the gap
#47111 hit ("only org owners can set controls") without per-request Owner
round-trips.

Related: #47111 (closed, stale), #68346 (prior attempt, closed), #48222
(extra-usage visibility on Team).