Add virtual portfolio filesystem commands

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Issue

Problem

The site is aiming toward richer terminal emulation, but filesystem commands were mostly placeholders: pwd always returned /home/guest, cd did not navigate, ls listed commands instead of files, and there was no way to inspect generated portfolio content as files.

Proposed improvement

Add a read-only virtual portfolio filesystem generated from the existing site data so visitors can navigate with familiar shell commands.

Changes

• Added virtual filesystem helpers for path normalization, node lookup, ls, cd, cat, and tree behavior.
• Wired terminal runtime commands so pwd, ls [path], cd [path], cat [path], and tree [path] operate on /home/guest.
• Preserved the command-list view behind ls --commands.
• Updated help/man content and README documentation.
• Added unit coverage for virtual path handling, directory listing, file reads, directory changes, and tree output.

Verification

• npm run check passed via the pre-push hook (lint, format check, coverage, build).