#276 portable readlink -f via abspath function

[VasilevNStas]

readlink -f is a GNU coreutils extension not available on macOS by default. This makes local development on macOS impossible — entry.sh immediately fails with "readlink: illegal option -- f".

What this PR does

Replaces the single readlink -f call with a portable abspath() function:

abspath() {
  local target="$1"
  while [ -L "${target}" ]; do
    target=$(readlink "${target}")
  done
  cd "$(dirname "${target}")" 2>/dev/null && pwd -P
}
self=$(abspath "$0")

Feature	Before	After
Linux (production)	✅ readlink -f	✅ abspath()
macOS (dev)	❌ illegal option -- f	✅ works
Symlink resolution	✅ one level	✅ chain (while loop)
Return type	absolute dir	absolute dir

Why not realpath?

realpath from brew install coreutils would require an external dependency. The cd + pwd -P approach is pure bash/POSIX and works everywhere.

Existing test coverage

The existing test test_resolves_real_path_when_invoked_via_symlink validates exactly this behaviour — it creates a symlink to entry.sh, invokes it, and asserts the resolved paths point to the real project directories.

Checklist

• bundle exec rubocop — 0 offences
• bundle exec rake — all tasks pass
• HoC ≤ 133

@yegor256 please review

[VasilevNStas]

@yegor256 — readlink -f is a GNU extension that doesn't exist on macOS. This means anyone developing on a Mac (including CI maintainers and contributors) gets an immediate error when testing entry.sh locally. The abspath() function replaces it with pure POSIX cd + pwd -P, preserving symlink resolution. The existing test test_resolves_real_path_when_invoked_via_symlink validates this behaviour. Low risk, high impact for developer experience.