docs(agents): add mandatory UAT rule + fix .crush prune in check-rules

- UAT rule: user must accept work before done-ratio 100% or close
- check-rules.sh: prune .crush/ from Discourse pointer scan (agent
  working space, not documentation)

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

scripts/check-rules.sh

@@ -157,7 +157,7 @@ while IFS= read -r -d '' f; do
         POINTER_MISSING=$((POINTER_MISSING + 1))
         $RULE_VERBOSE && printf '    %s\n' "$f"
     fi
-done < <(find . -path ./.git -prune -o -path ./.tmp -prune -o -path ./vendor -prune -o -path ./archive -prune -o -name '*.md' -print0 2>/dev/null)
+done < <(find . -path ./.git -prune -o -path ./.crush -prune -o -path ./.tmp -prune -o -path ./vendor -prune -o -path ./archive -prune -o -name '*.md' -print0 2>/dev/null)
 if [ "$POINTER_MISSING" -eq 0 ]; then
     check "All non-exempt .md cite Discourse ($DISCOURSE_HOST)" "pass"
 else