fix(quick): add gsd-sdk pre-flight check with install hint (#2334) (#2354)

Closes #2334

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/workflows/quick.md

@@ -124,6 +124,19 @@ If `$VALIDATE_MODE` only:
 
 **Step 2: Initialize**
 
+```bash
+if ! command -v gsd-sdk &>/dev/null; then
+  echo "⚠ gsd-sdk not found in PATH — /gsd-quick requires it."
+  echo ""
+  echo "Install the GSD SDK:"
+  echo "  npm install -g @gsd-build/sdk"
+  echo ""
+  echo "Or update GSD to get the latest packages:"
+  echo "  /gsd-update"
+  exit 1
+fi
+```
+
 ```bash
 INIT=$(gsd-sdk query init.quick "$DESCRIPTION")
 if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi