governance: add surface/writing lane (writing.kaihacks.ai archive shelf)

The writing surface's pages were unowned - no branch could legally touch
src/pages/writing/**. One explicit lane, same shape as the other surfaces.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0176ZYTwoV8V4So9SBTizc5X

Author: claude

khai-guard.config.json

@@ -117,6 +117,11 @@
         "layer": "solution",
         "allow": ["src/pages/misfits/**", "src/pages/main/misfits/**"]
       },
+      {
+        "pattern": "surface/writing/*",
+        "layer": "solution",
+        "allow": ["src/pages/writing/**"]
+      },
       {
         "pattern": "deps/*",
         "layer": "deps",