TLDR; Park outside.
Many years ago, a family I know was totally wiped out because of their car left running in the garage which was under the house.
There are a number of things to unpack in your question, but to me, the line quoted above is the key. A lot has changed since "many years ago." The most important of which are auto exhaust systems and building codes. The upside of this is that modern vehicles, especially those that are PZEVs, have reduced the overall pollutant (including carbon monoxide or CO) emissions at the tailpipe compared to vehicles of even 20 years ago. So it's quite possible you could leave your vehicle running in your garage for days with limited risk to members of the household.
Add to this building codes which were changed in the wake of tragedies such as the one you describe, and even if a deadly concentration of gas were to build up in a garage, if the house was built in the last decade, and was built properly, then sealed drywall joints and electrical boxes, weatherstripping on entry doors, and vapor barriers behind the drywall should prevent the ingress of fumes into the home.
So, should you leave a car running, the chances of it wiping out a family as it would have in 1980 are very small. Building upon this foundation, it seems likely that if you live in a modern house you could mitigate the risk to a low level simply by ventilating your garage. Thus, if your concern is really mitigating the risk of killing your family by leaving the car running, the answer is probably, in order of simplicity:
- Park outside. (I haven't parked a car in a garage in 35 years, even in New England winters.)
- Leave a window open in the garage.
- Adjust your garage door to leave a gap for ventilation.
- Install a constantly running ventilation fan to exhaust air from the garage.
As others have said, you can also install CO detectors in the home, but frankly, if you are parking after a long enough drive to have a hot catalytic converter you are likely producing more CO2 than CO so a detector would be less effective than you might think, but put one in the garage and one in the house anyway. One article on this phenomenon noted a state medical examiner who said, "If we find victims of this, there is never a CO detector."
If, however, your question is really a mental justification for how to work around the keyless ignition system and simply parking outside isn't acceptable, then you're into the realm of behavior modification and you really need to set up a system that guarantees success. You alluded to the house key being attached to the car key a feature. So perhaps you can adopt a method for putting your house key on top of the Subaru start button. Maybe you just glue a hook there and reaching for the key requires you to press the button. Maybe you add a big sign on the garage wall that asks "Is the car off?" But somehow you need to create a kind of interlock that prevents entry into the dwelling before pressing the button to turn the car off.