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/database-guidelines.md

@@ -0,0 +1,51 @@
+# Database Guidelines
+
+> Database patterns and conventions for this project.
+
+---
+
+## Overview
+
+<!--
+Document your project's database conventions here.
+
+Questions to answer:
+- What ORM/query library do you use?
+- How are migrations managed?
+- What are the naming conventions for tables/columns?
+- How do you handle transactions?
+-->
+
+(To be filled by the team)
+
+---
+
+## Query Patterns
+
+<!-- How should queries be written? Batch operations? -->
+
+(To be filled by the team)
+
+---
+
+## Migrations
+
+<!-- How to create and run migrations -->
+
+(To be filled by the team)
+
+---
+
+## Naming Conventions
+
+<!-- Table names, column names, index names -->
+
+(To be filled by the team)
+
+---
+
+## Common Mistakes
+
+<!-- Database-related mistakes your team has made -->
+
+(To be filled by the team)