feat!: remove foreach primitive

Users should use regular steps with Elixir's built-in enumeration tools
(Enum.map, Task.async_stream) for batch processing instead.

- Remove foreach macro from DSL
- Remove foreach execution from executor and step runner
- Remove foreach-related context functions (current_item, current_index)
- Delete foreach test file and guide
- Update integration tests to use Enum.map instead of foreach
- Update README batch processing example

Author: kasvith