ProjectStateStore.read_maintainers has no production reader #608

After #588 removes write_project/write_maintainers, ProjectStateStore.read_maintainers is the remaining protocol member with no production caller (wiring audit CORE-7). Its two implementations also disagree: the filesystem store derives entries from project.maintainer_entries, while the in-memory store returns [].

Either give it a reader (the maintainers context key is the obvious one), or remove it from the protocol and both stores, with a BREAKING entry.

Drafted with Claude Code; reviewed and posted by me.