Onset of first Diagnose

In this [paper](https://www.nature.com/articles/s41588-025-02396-8) they use the onset of first diagnose, as a proxy for the disease onset to split their cohort into early and late onset depression.

Currently, the Diagnose data structure of the Phenopacket is missing that field. If there will be an update to the schema, it would be a delight to include it.

Start by reading the Phenopacket schema's Diagnose definition and the linked paper to understand how onset of first diagnosis is used. Clarify whether the schema will be updated and what representation is intended; done means the requested information is specified and included in an agreed schema update.

Written by the indexing model from the issue text.