Switch packaging system from setuptools to UV-compatible hatchling

[Copilot]

This PR switches the packaging system from setuptools to hatchling (UV-recommended build backend) and adds installation instructions for UV tool.

Changes Made

Build System Migration

• pyproject.toml: Changed build-system from setuptools to hatchling
• Removed setuptools-specific configuration (tool.setuptools.packages.find)
• Added hatchling configuration (tool.hatch.build.targets.wheel)
• Fixed readme filename reference (README.md → readme.md)

Installation Instructions

• readme.md: Added UV as the recommended installation method
• Provided clear installation options for both UV and pip users
• Added development installation instructions with UV support

Development Environment

• shell.nix: Added UV package and switched from setuptools to hatchling
• Updated postShellHook to install hatchling instead of setuptools

Verification

• ✅ All existing tests continue to pass
• ✅ Package builds successfully with hatchling
• ✅ CLI tools (gpt-copy, tokens) work correctly
• ✅ Installation via pip still works
• ✅ Ready for UV tool installation

Installation Examples

Using UV (Recommended):

uv tool install git+https://github.com/simone-viozzi/gpt-copy.git

Using Pip:

pip install git+https://github.com/simone-viozzi/gpt-copy.git

Fixes #24.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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