refactor(core): restructure conversation/channel/message domains and modernize deployment

- Replace chat package with conversation flow architecture
- Add channel identity avatar support (migration 0002)
- Refactor channel adapters, identities, and message routing
- Update frontend: simplify composables, modernize UI components
- Improve Docker builds with cache mounts and version metadata
- Optimize healthchecks and simplify service dependencies

.gitignore

@@ -98,4 +98,5 @@ memory.db
 config.toml
 
 .workdocs/
-data
+data
+_main-ref/