Sub-DAG output variable propagation for parallel execution

## Problem

When a child sub-DAG sets `output:` on its steps, the parent's parallel results JSON shows `"outputs": {}` for every child. There is no way for a parent DAG to read collected outputs from parallel sub-DAG invocations.

## Expected Behavior

Child sub-DAG `output:` variables should be propagated to the parent and accessible from the parallel step's results. The parent should be able to reference aggregated outputs from all parallel children.

## Example

```yaml
# Parent
- id: fan_out
call: child-dag
parallel: ${ITEMS}
# After completion, ${fan_out.outputs} should be a JSON array
# of each child's output variables

- id: collect
depends: [fan_out]
command: echo "${fan_out.outputs}"
```

## Motivation

Fan-out/fan-in is a core DAG pattern. Without output propagation, the only way to pass data from parallel children back to the parent is through side-effect files (temp files on disk), which is fragile and hard to coordinate.

## Testing
- PR must include integration test under intg package

Start by tracing the parallel sub-DAG execution and result aggregation described in the issue, then inspect the intg package for existing integration-test patterns. Add coverage for a child step with output variables invoked through a parent parallel step. Done means the parent’s parallel results expose an aggregated JSON array that the following step can reference.

Written by the indexing model from the issue text.