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/quality-guidelines.md

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+# Quality Guidelines
+
+> Code quality standards for backend development.
+
+---
+
+## Overview
+
+<!--
+Document your project's quality standards here.
+
+Questions to answer:
+- What patterns are forbidden?
+- What linting rules do you enforce?
+- What are your testing requirements?
+- What code review standards apply?
+-->
+
+(To be filled by the team)
+
+---
+
+## Forbidden Patterns
+
+<!-- Patterns that should never be used and why -->
+
+(To be filled by the team)
+
+---
+
+## Required Patterns
+
+<!-- Patterns that must always be used -->
+
+(To be filled by the team)
+
+---
+
+## Testing Requirements
+
+<!-- What level of testing is expected -->
+
+(To be filled by the team)
+
+---
+
+## Code Review Checklist
+
+<!-- What reviewers should check -->
+
+(To be filled by the team)