Optional environment inheritance for sub-DAGs

## Problem

Sub-DAGs do not inherit any environment variables from their parent. Every shared value (`TODAY`, `SINCE`, `GH_USER`, etc.) must be explicitly passed as a param. For workflows with many shared variables, this creates verbose and fragile parameter lists that must be kept in sync between parent and child.

## Expected Behavior

An opt-in mechanism to inherit parent environment variables in sub-DAGs, either all or a specified subset.

## Example

```yaml
# Selective inheritance
- id: fan_out
call: child-dag
inherit_env: [TODAY, SINCE, GH_USER]
parallel: ${ORGS}

# Or inherit all
- id: fan_out
call: child-dag
inherit_env: true
parallel: ${ORGS}
```

## Motivation

Complex workflows often share configuration across parent and child DAGs (dates, usernames, API tokens, paths). Explicitly passing each one as a param is tedious and error-prone — adding a new shared variable requires updating both the parent's `params:` and the child's `params:` definition. This is the documented pitfall "Sub-DAGs do not inherit parent env vars" but could be solved with an opt-in mechanism that preserves the current safe default.

Start by tracing the sub-DAG call path described in the issue and how parent environment variables and params are handled. Define the behavior for inherit_env as a variable list or true while preserving the current default, then add tests covering selective inheritance, all-variable inheritance, and omitted inheritance.

Written by the indexing model from the issue text.