Fix #53: avatar clipped — switch to local 200x200 square image

Root cause: external GitHub URL (github.com/nikhilsi.png) served
inconsistently. Fix: download, crop to perfect 200x200 square,
host locally at docs/assets/images/nikhil-avatar.png. Existing
CSS fix (object-fit: cover) now works with a reliable source image.
Build tested locally.

Author: Claude-Ops

docs/assets/images/nikhil-avatar.png

@@ -2,4 +2,4 @@ authors:
   nikhil:
     name: Nikhil Singhal
     description: Builder who governs. Founder, AI Trust Commons. Author, Human Intelligence Partnership Charter.
-    avatar: https://github.com/nikhilsi.png
+    avatar: assets/images/nikhil-avatar.png