all: add GEMINI.md #19113

Job	Run time
race_dashboard	3m 51s
race	7m 35s
build	3m 13s
dashboard	2m 22s
old	6m 51s
aux	3m 55s
arch (presubmit_arch_freebsd)	5m 54s
arch (presubmit_arch_netbsd)	3m 54s
arch (presubmit_arch_windows)	3m 17s
arch (presubmit_arch_darwin)	3m 58s
gvisor	4m 3s
arch (presubmit_arch_executor)	3m 19s
arch (presubmit_arch_openbsd)	3m 55s
arch (presubmit_arch_linux)	10m 54s
	1h 7m 1s

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