Create System Template Child

Create a child template that inherits from a system-org parent.

Body:

• name (required, string, must be distinct from parent's name)
• description (optional, string)

The new child:

• inherits prompt / goals / outputs / language / phases from the parent (via the runtime resolver — nothing is copied at creation time)
• starts with empty settings.skills (admin adds extras after creation)
• status='draft' (admin publishes when ready)
• org_id = SYSTEM_ORG_ID (matches parent's org by definition)

Single-level invariant: parent must itself be a standalone template (parent.parent_template_id IS NULL). If the request targets a child as a parent, returns 400.