refactor: rebrand from cs-agent to BossChat

- Rename Go module: cs-agent → bosschat
- Rename all import paths across 200+ Go files
- Rename SDK: cs-ai-agent-sdk → bosschat-sdk
- Rename message protocol: cs-agent:* → bosschat:*
- Rename JS globals: CSAgentConfig → BossChatConfig
- Rename CSS classes: cs-agent-* → bosschat-*
- Update Makefile APP_NAME and ARCHIVE_NAME
- Update logo assets
- Fix display name: BoosChat → BossChat

web/styles/main.scss

@@ -4,7 +4,7 @@
 $thumb: #cbd5e1;
 $thumb-hover: #94a3b8;
 
-.cs-agent-scrollbar {
+.bosschat-scrollbar {
   scrollbar-width: thin;
   scrollbar-color: $thumb transparent;
 
@@ -29,7 +29,7 @@ $thumb-hover: #94a3b8;
   }
 }
 
-.dark .cs-agent-scrollbar {
+.dark .bosschat-scrollbar {
   scrollbar-color: hsl(var(--border)) transparent;
 
   &::-webkit-scrollbar-thumb {