Refactor environment and configuration management

- Updated the project structure to store configuration files (.env, models.yaml, extensions.yaml) in the project root instead of the user's home directory.
- Enhanced the setup process to automatically copy example files to the project root on first run.
- Implemented a migration function to transfer legacy configuration files from the user's home directory to the new project structure.
- Revised documentation to reflect changes in file locations and setup instructions.
- Improved code readability and maintainability by refactoring path management functions.

src/gpu_rent/ssh_keys.py

@@ -1,4 +1,4 @@
-"""Ed25519 key at ~/.gpu-rent/id_ed25519 (no passphrase)."""
+"""Ed25519 key at <project>/.gpu-rent/id_ed25519 (no passphrase)."""
 
 from __future__ import annotations