Add an `@fedify/lint` rule for actor preferred usernames

[dahlia]

Why

Most fediverse software expects actor objects to expose a stable preferredUsername. Fedify applications can omit it, but that tends to make remote display, search, and profile rendering worse. A lint rule can catch this during development without changing Fedify runtime behavior.

Scope

Add a new @fedify/lint rule that warns when an actor returned from setActorDispatcher() does not include preferredUsername. Follow the existing actor property rule patterns in packages/lint/src/rules/, packages/lint/src/lib/, and packages/lint/src/tests/.

The rule should recognize non-actor return values and avoid false positives in examples where the returned value is not a Fedify vocabulary actor.

Suggested checks

Run mise run check-each lint. If rule documentation is generated or maintained manually, update docs/manual/lint.md as well.

[Jae-Hyuk-Jang]

I’d like to take on this issue; could you assign it to me?

[dahlia]

@Jae-Hyuk-Jang Assigned. Thanks!