fix: prevent stdout duplication when capture_stdout is enabled

The previous implementation wrote command output to GITHUB_OUTPUT twice:
1. Via `tee -a "${GITHUB_OUTPUT}"` directly
2. Via the subshell redirect `} >>"${GITHUB_OUTPUT}"`

This caused every line of captured output to appear twice.

Fix: Change `tee -a "${GITHUB_OUTPUT}"` to `tee /dev/stderr` so that:
- Real-time output is still visible in the Actions log (via stderr)
- Output is only written once to GITHUB_OUTPUT (via subshell redirect)

entrypoint.sh

@@ -73,7 +73,7 @@ echo "======================================="
 if [[ "${INPUT_CAPTURE_STDOUT}" == 'true' ]]; then
   {
     echo 'stdout<<EOF'
-    "${TARGET}" "$@" | tee -a "${GITHUB_OUTPUT}"
+    "${TARGET}" "$@" | tee /dev/stderr
     echo 'EOF'
   } >>"${GITHUB_OUTPUT}"
 else