Remove a free agent from the pool

Withdraws a free agent from the tournament pool as the organizer and expires their pending offers. Requires the tournaments free_agents capability and MANAGER authority - a tournament MODERATOR qualifies. ORDERING: an agent whose status is PICKED_UP is refused with 400 error_invalid_input ("Remove the player from the team roster first") and nothing is written, because withdrawing them while their FREE_AGENT roster entry survives would strand the player - no re-entry path accepts a non-AVAILABLE pool row, and the surviving roster entry keeps the re-pickup guards rejecting. Delete that roster entry first via DELETE /tournaments/{identifier}/roster-entries/{rosterEntryId}, which releases the agent back to AVAILABLE, then call this endpoint. An AVAILABLE agent can be removed directly.