refactor: replace custom PEFT pipeline with Transformers+PEFT example

Remove the custom peft/ directory (LoRA/DoRA adapters, trainer, data
pipeline) in favor of a lightweight fine-tuning example that uses the
standard HuggingFace Transformers + PEFT ecosystem.

The new example at timesfm-forecasting/examples/finetuning/ demonstrates
LoRA fine-tuning via TimesFm2_5ModelForPrediction and the peft library,
based on the approach by @kashif at HuggingFace.

- Remove peft/ (8 files)
- Add timesfm-forecasting/examples/finetuning/finetune_lora.py
- Add timesfm-forecasting/examples/finetuning/README.md
- Update README.md to reference new example
- Clean up .gitignore (remove peft_checkpoints/)

.gitignore

@@ -3,7 +3,6 @@ dist/
 __pycache__/
 *.egg-info/
 checkpoints/
-peft_checkpoints/
 wandb/
 datasets/
 results/