fix: step-8 last xargs instance → pure-bash whitespace strip

wc -c output piped through xargs interpreted backslashes in edge-case
byte counts; use parameter expansion instead (matches step-6/7 pattern).

Author: fidgetcoding

step-8/step-8-install.sh

@@ -244,7 +244,7 @@ run_self_test() {
 
     # Test 3: Skill file is non-empty
     if [ -s "$SKILL_FILE" ]; then
-        SKILL_SIZE=$(wc -c < "$SKILL_FILE" | xargs)
+        SKILL_SIZE=$(wc -c < "$SKILL_FILE"); SKILL_SIZE="${SKILL_SIZE//[[:space:]]/}"
         success "TEST: SKILL.md is non-empty ($SKILL_SIZE bytes)"
         TEST_PASS=$((TEST_PASS + 1))
     else