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/mod.rs

@@ -15,7 +15,9 @@ pub mod registry;
 pub mod types;
 
 pub use content_registry::ContentEntityRegistry;
-pub use culture::{CultureError, CultureResolver, CultureResolverResource, CultureTag, resolve_culture};
+pub use culture::{
+    resolve_culture, CultureError, CultureResolver, CultureResolverResource, CultureTag,
+};
 pub use events::{
     ContradictionDetectedEvent, ContradictionDetectedQueue, InteractionType, KnowledgeEvent,
     KnowledgeEventQueue, KnowledgeEventType, ProcessedEntityGrant, ProcessedFactGrant,