docs(#221): align --skip-gitignore help text with actual behavior

[kreinba]

Closes #221.

The --skip-gitignore flag's help text in bin/pdd:38 claimed Don't look into .gitignore for excludes, but the code in bin/pdd:71-78 does the opposite: when the flag is set and .gitignore exists, it reads the file and folds its entries into the excludes set. Multiple users (php-coder, proofit404, avidrucker) hit this contradiction, and pnatashap suggested clarifying the description on the issue thread.

The change rewrites the help string to describe the actual behavior. The flag name itself is unchanged so no downstream invocation breaks.

Ran ./bin/rake test and ./bin/rubocop bin/pdd; the change introduces no new offenses or test failures. Pre-existing failures in test_skips_uncommitted_changes and test_reads_git_author are unrelated git-signing environmental errors.

[edmoffo]

Help string now matches the code path in bin/pdd:71-78, where --skip-gitignore reads .gitignore and folds its non-comment entries into the excludes set. Diff is one line, no code paths touched, no invariants at risk, no tests needed for a help-text change. The yamllint red comes from a missing newline in .github/workflows/actionlint.yml that the diff does not touch. No inline comments.