docs: polish documentation with accurate options, safety details, and test framework

[ashishsinghbora]

Summary

• Updated README.md to accurately reflect all 34 utilities, flags, options, and behaviors.
• Documented Alpine Linux (apk) support in distribution list.
• Documented bin/linux-toolbox CLI subcommands (list, run, direct execution) alongside the interactive menu.
• Added comprehensive section on Safety & Security Engineering (archive traversal security, backup recursion prevention, process termination safeguards, SSH key overwrite backups, and array execution).
• Documented the 53-assertion functional test suite and the multi-distro CI container matrix (Ubuntu, Fedora, Arch, Alpine).

Verification

• Verified Markdown links, table formatting, and code snippets.
• Verified test suite passes.

[ashishsinghbora]

Code review: verified accurate documentation, testing guidelines, and security engineering section.