Add `AGENTS.md` for AI coding agents #5997

Job	Run time
check-ubuntu-24_04-make-gcc	2m 23s
Emscripten build	1m 50s
check-macos-15-make-clang	3m 10s
check-ubuntu-24_04-make-clang	2m 54s
CentOS 8	1m 7s
check-vs-2022-make-build-and-test	2m 24s
benchmarking	3m 36s
check-ubuntu-24_04-make-clang-KNOWNBUG	47s
ebmc-spot	23s
examples	7s
	18m 41s

Provide feedback