feat(engine): retire D-078 overheard conversation system (#848)

Per R-012: delete conversation.rs, both overheard content files, and
remove all 6 wire-up points (social_plugin, bridge/types, monologue,
voice/integration). Protocol version 22 → 23. Scope confirmed by
#842 audit — npc/ and content/global/ untouched. Surviving NPC
components (NpcName, NpcColorIndex, NpcConversation) migrated to
simulation/npc_components.rs for use by D-080 knowledge propagation.
Also applies pre-existing cargo fmt debt (names.rs and 4 others).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/knowledge/events.rs

@@ -185,7 +185,7 @@ pub fn process_knowledge_events(
     mut queue: ResMut<KnowledgeEventQueue>,
     mut contradiction_queue: ResMut<ContradictionDetectedQueue>,
     registry: Res<EntityRegistry>,
-    npc_names: Query<&crate::simulation::conversation::NpcName>,
+    npc_names: Query<&crate::simulation::npc_components::NpcName>,
     mut knowledge_query: Query<&mut KnowledgeGraph>,
 ) {
     let events = queue.drain();