Guard Stop hook typecheck against missing react-router binary

In remote execution environments where node_modules is not installed,
npm run typecheck fails with "react-router: not found" rather than a
real TypeScript error. This caused the Stop hook to loop the session
indefinitely. The guard exits cleanly when react-router is absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KiVGo8gSBXe3WuRniVNKsd

.claude/settings.json

@@ -18,7 +18,7 @@
         "hooks": [
           {
             "type": "command",
-            "command": "output=$(npm run typecheck 2>&1); rc=$?; if [ $rc -ne 0 ]; then printf '{\"systemMessage\":\"TypeCheck failed:\\n%s\"}' \"$(echo \"$output\" | tail -30 | sed 's/\"/\\\\\"/g; s/$/\\\\n/' | tr -d '\\n')\"; fi",
+            "command": "which react-router > /dev/null 2>&1 || exit 0; output=$(npm run typecheck 2>&1); rc=$?; if [ $rc -ne 0 ]; then printf '{\"systemMessage\":\"TypeCheck failed:\\n%s\"}' \"$(echo \"$output\" | tail -30 | sed 's/\"/\\\\\"/g; s/$/\\\\n/' | tr -d '\\n')\"; fi",
             "timeout": 60
           }
         ]