Add preservation of site-packages directories on upgrade.

[zooba]

No description provided.

[Copilot]

Pull Request Overview

This PR adds functionality to preserve site-packages directories during Python installation upgrades, preventing the loss of user-installed packages when upgrading Python versions.

• Introduces site-packages preservation logic with _preserve_site and _restore_site functions
• Adds configuration option preserve_site_on_upgrade with default value True
• Integrates preservation workflow into the installation process with proper cleanup

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/manage/install_command.py	Core implementation of site-packages preservation and restoration logic
src/manage/commands.py	Configuration option and command class attribute for preservation feature
tests/test_install_command.py	Comprehensive test coverage for preservation functionality

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