Rowan is probably right, but you can try to get the crankarm tight enough to see if it will work. If you were not pedaling hard, you might not have damaged it. It might have had a squeaking noise before it got really loose, that is another warning to be aware of. I have bought cranksets on ebay for under $30 shipped, watch for a replacement, but you would have to know what type of bottom bracket you want to put the crankset on.

Regarding your specific question, how to carry tools and you mentioned in the bottle cage - I have a plastic bottle that was sold for that purpose, fits perfectly in the cage and I carry tools, saddle rain cover, plastic bag (for under rain cover for redundancy), some plastic disposable gloves from my dentist's office to keep my hands clean when I do a repair, spare tube, tire levers, etc. I also have a nylon bag that was designed to fit in the cage and have used that too. Some peanut butter plastic jars will fit fine, as do some parmesan cheese plastic bottles. If you use a food jar, consider painting it first so people don't see your tools in the jar.

I aways pack my took kit pretty full and it is hard to dig out a small tool when I need it so I also carry a small multi-tool in my pocket on rides farther than a mile or so. The multi-tool I usually carry in the pocket does not have the 8mm bold most crankarms use, but it will work on everything else. The multi-tool in the took kit includes that 8mm bolt.

I have been working with tools for decades and rarely use a torque wrench, but the crankset bolt that holds the crank arm to the bottom bracket is the one bolt on a bike that I always use a torque wrench on. I had to buy the 1/2 inch drive 8mm allen tool for that specific bolt.

Crank arm bolts usually come with blue loctite, if yours did not you might want to buy some.