fix(people-lists): preserve data written by other Nostr clients #8729

Summary

Editing a NIP-51 people list rebuilds the complete kind-30000 event from UserList, deleting tag names, tag positions, and encrypted content the model does not represent.

Steps to Reproduce

1. Read a kind-30000 people-list event containing an unmodelled tag, a p tag with a relay hint or other later position, or encrypted private items in content.
2. Decode it through Nip51PeopleListCodec.decode.
3. Publish an add/remove edit through PeopleListsRepositoryImpl, which calls Nip51PeopleListCodec.encode.
4. Inspect the replacement event.

Actual Result

encode creates a new array containing only d, title, optional description/image, and minimal p tags. It also emits empty content. Metadata and private list items written by another Nostr client are silently removed from the parameterized-replaceable event.

Expected Result

A membership edit should preserve every untouched source tag, every untouched tag position, and the source content verbatim. Only the member being added or removed should change.

Regression?

Unknown. This is a latent interoperability and data-preservation defect in the current codec.

Evidence

mobile/packages/people_lists_repository/lib/src/nip51_people_list_codec.dart rebuilds the publish payload from UserList; mobile/packages/people_lists_repository/lib/src/people_lists_repository_impl.dart publishes that payload as the replacement event.

Discovered while applying the same preservation rule to kind 10003 in PR #8721.