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/directory-structure.md

@@ -0,0 +1,54 @@
+# Directory Structure
+
+> How backend code is organized in this project.
+
+---
+
+## Overview
+
+<!--
+Document your project's backend directory structure here.
+
+Questions to answer:
+- How are modules/packages organized?
+- Where does business logic live?
+- Where are API endpoints defined?
+- How are utilities and helpers organized?
+-->
+
+(To be filled by the team)
+
+---
+
+## Directory Layout
+
+```
+<!-- Replace with your actual structure -->
+src/
+├── ...
+└── ...
+```
+
+---
+
+## Module Organization
+
+<!-- How should new features/modules be organized? -->
+
+(To be filled by the team)
+
+---
+
+## Naming Conventions
+
+<!-- File and folder naming rules -->
+
+(To be filled by the team)
+
+---
+
+## Examples
+
+<!-- Link to well-organized modules as examples -->
+
+(To be filled by the team)