Get Credits

Current credit balance plus the purchasable packs.

Also echoes this account's own referral_code/referral_url -- the "narrow ambient" surface for the referral program (see mcp_server/tools.py's get_referral_link for the pull surface). An agent that's already calling check_balance for its human learns the code for free, no extra round trip, and with no pitch text attached: this is pure account state, the same as balance itself. Nothing else about this endpoint changes.