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/scripts/common/io.py

@@ -0,0 +1,61 @@
+"""
+JSON file I/O utilities.
+
+Provides read_json and write_json as the single source of truth
+for JSON file operations across all Trellis scripts.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import tempfile
+from pathlib import Path
+
+
+def read_json(path: Path) -> dict | None:
+    """Read and parse a JSON file.
+
+    Returns None if the file doesn't exist, is invalid JSON, or can't be read.
+    """
+    try:
+        return json.loads(path.read_text(encoding="utf-8"))
+    except (FileNotFoundError, json.JSONDecodeError, OSError):
+        return None
+
+
+def write_json(path: Path, data: dict) -> bool:
+    """Write dict to JSON file with pretty formatting.
+
+    The write is atomic: content goes to a temp file in the same directory
+    and is then renamed over the target. A crash or Ctrl-C mid-write leaves
+    the existing file intact rather than truncated, so a corrupted task.json
+    can never make a task silently vanish from `task.py list`.
+
+    Returns True on success, False on error.
+    """
+    payload = json.dumps(data, indent=2, ensure_ascii=False)
+    try:
+        fd, tmp = tempfile.mkstemp(
+            dir=str(path.parent), prefix=f".{path.name}.", suffix=".tmp"
+        )
+    except OSError:
+        return False
+
+    try:
+        try:
+            f = os.fdopen(fd, "w", encoding="utf-8")
+        except OSError:
+            # fdopen never took ownership of fd; close it ourselves.
+            os.close(fd)
+            raise
+        with f:
+            f.write(payload)
+        os.replace(tmp, path)
+        return True
+    except OSError:
+        try:
+            os.unlink(tmp)
+        except OSError:
+            pass
+        return False