test: add comprehensive isolated functional test suite and harden toolbox CLI

[ashishsinghbora]

Summary

Tests

• Added tests/functional_test.sh providing 53 isolated behavioural and security assertions across all script categories.
• Covers path traversal and absolute path blocking, backup exclusion of destination directory, file organizer collisions/symlink handling, mock package managers (pacman, apt, dnf, zypper, apk), safe process termination, and SSH key generation/backup.
• Integrated tests/functional_test.sh into tests/test_all.sh for automated execution.

Enhancements & Bug Fixes Discovered by Test Suite

• bin/linux-toolbox: added CLI subcommands (list, run, and direct script name execution) while preserving the interactive keyboard menu.
• scripts/process/process-info.sh: added PID existence checking and status reporting, process name searching via pgrep, user filtering, and top CPU/memory summaries.
• scripts/storage/directory-size.sh: supported -d / --dir flags in addition to positional arguments.
• scripts/system/kernel-info.sh: fixed SIGPIPE (exit 141) in zcat /proc/config.gz | head pipeline under pipefail.

Verification

• Verified all 53 functional tests pass cleanly.
• Verified ShellCheck passes without warnings.
• Verified tests/test_all.sh completes with 0 errors.

[ashishsinghbora]

Code review: verified 53 end-to-end isolated behavioral and security tests covering all script categories, toolbox CLI subcommands, and graceful handling of unbooted systemd environments.