Add a creator

Add a creator to a plan awaiting approval by its unit_id (a format's unit_id from creators_search or creator_get), at that format's current all-in price, the same as the brand adding it in the workspace. Leave out plan_id (REST: POST /api/v1/plans/current/lines) to add to the brand's newest plan awaiting approval, or to start a plan for this creator when none waits; that also needs the mandate to allow starting a campaign, and counts toward the brand's 3 new campaigns in 24 hours. Only approved creators on sale can be added (not_approved otherwise). A creator who would take the plan's all-in total over the mandate's maximum plan total, or this month's signed spend plus that total over its monthly cap, is refused (mandate_refused, over_max_plan_total or over_monthly_cap) and the plan is left as it was. A creator who would take the plan's all-in total over its campaign's budget is refused for everyone (conflict, reason over_budget, with budget_cents and remaining_cents, the room left before this creator) and the plan is left as it was; pick a creator priced within remaining_cents or remove one first. Returns the new line_id and the plan as plan_quote returns it. Needs an idempotency_key. Scope: campaigns:write. MCP tool: plan_add_creator.