fix(ln): `ln -s` blocked by RealFs sandbox — no symlink support

[chaliy]

Summary

`ln -s TARGET LINKNAME` fails with: `symlink creation is not allowed in RealFs (sandbox security)`

Even with `--mount-rw`, symlink creation is blocked.

Reproduction

# With --mount-rw /tmp:/tmp
echo "body{}" > /tmp/style.css
ln -s /tmp/style.css /tmp/style_link.css
# Error: symlink creation is not allowed in RealFs (sandbox security)

Context

Bashblog creates a symlink when a parent page style exists:

if [[ -f ../style.css ]] && [[ ! -f main.css ]]; then
    ln -s "../style.css" "main.css"
fi

This is a minor issue since the fallback path creates a full main.css file instead.

Expected behavior

When a directory is mounted with `--mount-rw`, symlink creation should be allowed within that mount. Alternatively, provide a clear `--allow-symlinks` flag.