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.

.gitattributes

@@ -0,0 +1,9 @@
+# Trellis: append-only developer journals should merge cleanly across
+# parallel sessions/worktrees — each session only appends a new block, so
+# there is nothing to actually conflict on.
+#
+# Do NOT add a rule for workspace/*/index.md here — it is fully regenerated
+# every session, so a real conflict there is expected and safe to resolve by
+# picking either side (task state lives in task.json, not index.md). See
+# .trellis/spec/cli/backend/directory-structure.md for details.
+.trellis/workspace/*/journal-*.md merge=union