Set Composer 2.5 (Grok Build TUI) effort to Default, not blank

A missing effort reads as Max by the repo's "effort defaults to max"
convention; this run was the harness default, so set it explicitly to
"Default" and surface it in both README registry rows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/composer-2-5-grok-build.json

@@ -2,6 +2,7 @@
   "label": "Composer 2.5",
   "vendor": "Cursor",
   "harness": "Grok Build TUI",
+  "effort": "Default",
   "order": 72,
   "runs": {
     "mythos-craft": {