Read a project's current product goal (the outcome work is aimed at).
READ THIS BEFORE YOU CALL `set_product_goal`: the setter REPLACES the whole
document rather than appending to it, so writing without reading first
silently discards whatever the user already recorded. To add a line, read
the current text, edit it, and set the full result back.
Returns {project_id, product_goal_md, updated_at}. `product_goal_md` is
None when no goal has been set. Tenant-scoped: a project not in the
caller's workspace 404s.
ConnectorNo auth