Create a Discovery Agent creator profile

Create a new creator brief for the shop — a search_query plus optional demographic filters — and resolve it immediately, so the response carries the creator_count it matched.

The brief is created with source: user_created and shows up in GET /discovery/profiles right away. The stored title is regenerated from the query and filters rather than kept verbatim, so read it back from the response instead of assuming what you sent.

Requires read_write scope and an Idempotency-Key header — a replay of the same key and body returns the original 201 rather than creating a second brief. Single-shop only.