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.

.pi/settings.json

@@ -0,0 +1,9 @@
+{
+  "enableSkillCommands": true,
+  "extensions": [
+    "./extensions/trellis/index.ts"
+  ],
+  "prompts": [
+    "./prompts"
+  ]
+}