Add workspace index and developer-specific journals for AI sessions

- Created a main workspace index to track AI Agent work records across developers.
- Added a personal index for developer 'yuxuanhui' with current status and session history.
- Initialized the first journal file for 'yuxuanhui' to document AI development sessions.
- Introduced AGENTS.md for Trellis instructions, detailing project structure and usage guidelines.

.trellis/spec/backend/error-handling.md

@@ -0,0 +1,51 @@
+# Error Handling
+
+> How errors are handled in this project.
+
+---
+
+## Overview
+
+<!--
+Document your project's error handling conventions here.
+
+Questions to answer:
+- What error types do you define?
+- How are errors propagated?
+- How are errors logged?
+- How are errors returned to clients?
+-->
+
+(To be filled by the team)
+
+---
+
+## Error Types
+
+<!-- Custom error classes/types -->
+
+(To be filled by the team)
+
+---
+
+## Error Handling Patterns
+
+<!-- Try-catch patterns, error propagation -->
+
+(To be filled by the team)
+
+---
+
+## API Error Responses
+
+<!-- Standard error response format -->
+
+(To be filled by the team)
+
+---
+
+## Common Mistakes
+
+<!-- Error handling mistakes your team has made -->
+
+(To be filled by the team)