Rename express/ to backend/ and update references

Update paths in sync.sh, master/main.go, and CLAUDE.md to reflect
the directory rename.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -38,7 +38,7 @@ master process. Key design principles:
 
 **Format TypeScript code:**
 ```bash
-cd express && ../cmd pnpm biome check --write .
+cd backend && ../cmd pnpm biome check --write .
 ```
 
 **Build Go master process:**
@@ -54,7 +54,7 @@ cd master && go build
 
 ### Components
 
-- **express/** - TypeScript/Express.js backend application
+- **backend/** - TypeScript/Express.js backend application
 - **master/** - Go-based master process for file watching and process management
 - **framework/** - Managed binaries (Node.js, pnpm), command wrappers, and
   framework-specific library code
@@ -68,7 +68,7 @@ Responsibilities:
 - Proxy web requests to backend workers
 - Behaves identically in all environments (no dev/prod distinction)
 
-### Express App Structure
+### Backend App Structure
 
 - `app.ts` - Main Express application setup with route matching
 - `routes.ts` - Route definitions