Add host-env tool

[midwan]

Summary

• Add host-env for getting, setting, unsetting, and listing host user environment variables.
• Persist Windows values through the user environment store, and Linux/macOS values through $HOME/.host-tools-env.
• Wire the new tool into the build, installer, package layout, README, and AmigaGuide docs.
• fixes Add a tool for getting/setting Env variables #12

Test Plan

• make -B tests/test_host_common.out tests/test_host_command_builders.out tests/test_host_edit_command.out tests/test_host_download_command.out && ./tests/test_host_common.out && ./tests/test_host_command_builders.out && ./tests/test_host_edit_command.out && ./tests/test_host_download_command.out
• sh tests/test_ahi_driver_source.sh
• git diff --check HEAD~1..HEAD
• docker run --rm -v "$PWD":/work -w /work sacredbanana/amiga-compiler:m68k-amigaos make -B host-env
• docker run --rm -v "$PWD":/work -w /work sacredbanana/amiga-compiler:m68k-amigaos sh tests/test_package_layout.sh

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