## What changed
- Add a dedicated contextual fragment for developer instructions that emits the
`generic.developer_instructions` content kind.
- Use the fragment when adding child-specific developer instructions to a
full-history subagent fork, preserving their classification and ensuring they
reach the child exactly once without appearing in the parent request.
## Testing
- Extend full-history subagent coverage to verify the child-only instructions'
annotation, placement, and count.
GitOrigin-RevId: 82589d311535225fed76d500a19458fda72f40f2