Draft journeys for a persona

Exactly ONE grounding source: serverAttachmentId or environmentId. Sending both is a 400 — a request that is already confused about what it wants should not have a winner picked for it.

DRAFTS. Nothing here is persisted — feed what you want to keep to POST /projects/{projectId}/personas or POST /projects/{projectId}/journeys.

The shape depends on the request: POST /personas/generate returns {persona, journeys} without personaCount and {personas: [{persona, journeys}]} with it; POST /journeys/generate returns {journeys}.