all: add GEMINI.md #19082

Job	Run time
aux	4m 0s
old	7m 15s
race_dashboard	3m 13s
race	7m 13s
dashboard	2m 45s
build	4m 6s
gvisor	4m 7s
arch (presubmit_arch_linux)	10m 39s
arch (presubmit_arch_freebsd)	6m 26s
arch (presubmit_arch_openbsd)	3m 53s
arch (presubmit_arch_darwin)	3m 45s
arch (presubmit_arch_executor)	3m 14s
arch (presubmit_arch_windows)	3m 15s
arch (presubmit_arch_netbsd)	3m 56s
	1h 7m 47s

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