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/frontend/state-management.md

@@ -0,0 +1,51 @@
+# State Management
+
+> How state is managed in this project.
+
+---
+
+## Overview
+
+<!--
+Document your project's state management conventions here.
+
+Questions to answer:
+- What state management solution do you use?
+- How is local vs global state decided?
+- How do you handle server state?
+- What are the patterns for derived state?
+-->
+
+(To be filled by the team)
+
+---
+
+## State Categories
+
+<!-- Local state, global state, server state, URL state -->
+
+(To be filled by the team)
+
+---
+
+## When to Use Global State
+
+<!-- Criteria for promoting state to global -->
+
+(To be filled by the team)
+
+---
+
+## Server State
+
+<!-- How server data is cached and synchronized -->
+
+(To be filled by the team)
+
+---
+
+## Common Mistakes
+
+<!-- State management mistakes your team has made -->
+
+(To be filled by the team)