Claim a credit invite link for your own balance [write]

Redeems an operator's credit invite link (agentsky.dev/i/<code>) the way the invite page's Claim button does; code is the <code> part of the link. Not a workspace invitation: those are POST /invites/{token}. 201 granted, or 200 already_claimed when this account redeemed the link before (nothing is funded twice). 404 not_found for an unknown code; 410 invite_expired, invite_disabled or invite_used; 403 invite_not_yours for a personal link issued to another address, 403 account_ineligible for an unverified or banned account. Rate-limited per account (429 rate_limited with Retry-After). A workspace key or a restricted credential cannot claim credit.