migration: backfill links to migrated pages

## Summary

Some book content references other chapters that aren't yet migrated. These internal links need to be added once the target pages are migrated.

During migration, leave placeholder text or omit the link. After migration is complete, backfill all internal cross-references.

## Affected Pages

- [ ] [Shell Basics - Copying a File](https://effective.sh/appendices/shell-basics/#copying-a-file)

Open the affected Shell Basics page at the linked URL and check whether the target page has been migrated. Review its internal references and add the missing cross-reference once migration is complete. Done means the listed migrated-page link is present and no placeholder remains for that reference.

Written by the indexing model from the issue text.