Rename the experiment from SCOPED_INCLUDES to SCOPED_TASKFILES to better
reflect its expanded scope. This experiment now provides:
1. Variable scoping (existing): includes see only their own vars + parent vars
2. Environment namespace (new): env vars accessible via {{.env.XXX}}
With TASK_X_SCOPED_TASKFILES=1:
- {{.VAR}} accesses vars only (scoped per include)
- {{.env.VAR}} accesses env (OS + Taskfile env:, inherited)
- {{.TASK}} and other special vars remain at root level
This is a breaking change for the experimental feature:
- {{.PATH}} no longer works, use {{.env.PATH}} instead
- Env vars are no longer at root level in templates
When SCOPED_INCLUDES experiment is enabled:
- Resolve vars from DAG instead of merged vars
- Apply root Taskfile vars first (inheritance)
- Then apply task's source Taskfile vars from DAG
- Apply IncludeVars passed via includes: section
- Skip IncludedTaskfileVars (contains parent's vars, not source's)
This ensures tasks in included Taskfiles see:
1. Root vars (inheritance from parent)
2. Their own Taskfile's vars
3. Vars passed through includes: section
4. Call vars and task-level vars
