chore: import upstream snapshot with attribution

cli-anything-plugin/guides/session-locking.md

@@ -0,0 +1,39 @@
+# Session File Locking
+
+When saving session JSON, use exclusive file locking to prevent concurrent writes from corrupting data.
+
+## The Problem
+
+Never use bare `open("w") + json.dump()` — `open("w")` truncates the file before any lock can be acquired.
+
+## The Solution: `_locked_save_json`
+
+Open with `"r+"`, lock, then truncate inside the lock:
+
+```python
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")            # no truncation on open
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")             # first save — file doesn't exist yet
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass                        # Windows / unsupported FS — proceed unlocked
+        try:
+            f.seek(0)
+            f.truncate()                # truncate INSIDE the lock
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+```
+
+Copy this pattern into `core/session.py` for all session saves.