Simplify module.yaml and enhance export command with real export targets

- Remove redundant fields from module.yaml files: exports_to, files, components
- Keep only essential system metadata: name, title, description, dependencies
- Export command now reads actual export targets from dev files (#| default_exp directive)
- Status command updated to use dev files as source of truth for export targets
- Export command shows detailed source → target mapping for better clarity
- Dependencies field retained as it's useful for CLI module ordering and prerequisites
- Eliminates duplication between YAML and dev files - dev files are the real truth

tinytorch/core/activations.py

@@ -3,9 +3,6 @@
 # %% auto 0
 __all__ = ['ReLU', 'Sigmoid', 'Tanh']
 
-from tinytorch.core.tensor import Tensor
-import numpy as np
-
 # %% ../../modules/activations/activations_dev.ipynb 5
 class ReLU:
     """