keys abstract method
List keys, optionally filtered by prefix.
prefix is a string prefix matched per key with String.startsWith,
NOT a directory / path segment: a key is included iff
key.startsWith(prefix) (all keys when prefix is null or empty).
Implementations backed by a hierarchical store (e.g. a file tree) must
reconstruct full keys and filter by string prefix — they must NOT scope
the scan to a <root>/<prefix>/ directory, which silently drops flat
namespaced keys (e.g. philosophy.ethos:<id>) and partial-segment
prefixes. The in-memory reference implementation defines this contract.
Implementation
Future<List<String>> keys({String? prefix});