Add API-key-protected local Roblox backup copier

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Motivation

• Provide a safe, local CLI for backing up Roblox place files (.rbxl / .rbxlx) that the user already exported, with simple access control.
• Avoid storing plaintext secrets by using a generated API key and hashed-on-disk storage for authentication.
• Make the workflow reproducible and testable with a small unit test suite.

Description

• Add roblox_game_copier.py, a CLI that implements init-key to generate a key, hashed storage (.gamecopier_api_key by default), copy to validate an API key, and a timestamped backup of a local .rbxl/.rbxlx file.
• Implement helper functions generate_api_key, hash_key, save_key, load_key_hash, validate_api_key, and copy_game_file and a parse_args-driven command-line interface.
• Update README.md with quick-start usage for init-key and copy, and add test instructions.
• Add test_roblox_game_copier.py with unit tests for key generation/validation and the backup copy behavior.

Testing

• Ran the unit test suite with python3 -m unittest -v, which executed test_generate_and_validate_key and test_copy_game_file.
• Both tests passed and the test run completed with OK.

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Codex Task