fix: cross-platform browser opening and hook robustness

bin/amc:
- Add cross-platform browser opening: try 'open' (macOS) first,
  fall back to 'xdg-open' (Linux) for desktop environments
- Wrap browser opening in conditional to avoid errors on headless systems

bin/amc-hook:
- Fix edge case in question extraction where tool_input.get("questions")
  could return None instead of empty list, causing TypeError

bin/amc

@@ -34,7 +34,12 @@ case "$cmd" in
         echo "AMC started (pid $!)"
       fi
     fi
-    open "http://127.0.0.1:$PORT"
+    # Open browser (platform-specific)
+    if command -v open &>/dev/null; then
+      open "http://127.0.0.1:$PORT"
+    elif command -v xdg-open &>/dev/null; then
+      xdg-open "http://127.0.0.1:$PORT"
+    fi
     ;;
   stop)
     if is_running; then

bin/amc-hook

@@ -73,7 +73,7 @@ def _extract_questions(hook):
     if not isinstance(tool_input, dict):
         return []
 
-    questions = tool_input.get("questions", [])
+    questions = tool_input.get("questions") or []
     result = []
     for q in questions:
         entry = {