Correct stderr capture in test_skill_validation.sh

- Fixed Bash redirection bug () that prevented capturing stderr output.
- Replaced with  which correctly captures stderr (since stdout is empty on failure).
- Ensures validation checks for clean error messages and non-zero exit codes work as intended.

tests/test_skill_validation.sh

@@ -12,7 +12,7 @@ description: No closing
 SKILL
 
 set +e
-STDERR_OUTPUT=$("${DRHARNESS}" -D -F dummy.chat 2>&1 >/dev/null)
+STDERR_OUTPUT=$("${DRHARNESS}" -D -F dummy.chat 2>&1)
 EXIT_CODE=$?
 set -e
 
@@ -45,7 +45,7 @@ This skill has no frontmatter.
 SKILL
 
 set +e
-STDERR_OUTPUT=$("${DRHARNESS}" -D -F dummy.chat 2>&1 >/dev/null)
+STDERR_OUTPUT=$("${DRHARNESS}" -D -F dummy.chat 2>&1)
 EXIT_CODE=$?
 set -e