A configuration file may specify an extends key pointing to another file. The
referenced file is loaded first and the current file's values override it. The
path is resolved relative to the file containing the extends directive.
Missing files raise a not-found error that includes both the resolved absolute
path and the file that declared extends, making it clear what needs to be
created. Precedence across all sources becomes base file → extending file →
environment variables → CLI flags. Cycles are detected and reported via a
CyclicExtends error. Prefix handling and subcommand namespaces work as normal
when inheritance is in use.