.github: add basic copilot.md pointing to .agent/rules #22874

Job	Run time
LP64-WIP	3m 31s
manifest-check	2m 36s
build-windows (mnfst, ptl)	10m 57s
build-windows (-d, mnfst, lnl)	8m 35s
build-windows (mnfst, imx8m imx8ulp)	10m 16s
build-windows (mnfst, lnl)	9m 46s
build-windows (mnfst, mtl)	8m 46s
build-linux (mnfst, imx8 imx8x)	5m 5s
build-linux (mnfst, ptl)	7m 20s
build-linux (mnfst, tgl-h)	4m 42s
build-windows (mnfst, tgl)	13m 38s
build-linux (mnfst, lnl)	4m 37s
build-linux (mnfst, tgl)	4m 35s
build-linux (mnfst, imx8m imx8ulp)	5m 28s
build-windows (mnfst, tgl-h)	9m 47s
build-windows (mnfst, imx8 imx8x)	9m 36s
build-linux (mnfst, mtl)	4m 49s
build-linux (mnfst, wcl imx95)	7m 31s
build-linux (-d, mnfst, lnl)	4m 43s
compare-linux-win	36s
	2h 16m 54s

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