Add coding agent instructions: refuse PRs unless maintenance mode is acknowledged #35541

Job	Run time
format	28s
typecheck	44s
browser-integration	1m 42s
coverage	10m 49s
knip	31s
misc	29s
Test Node 24 on ubuntu-latest	8m 34s
self-check	53s
Test Node lts/* on ubuntu-latest with --no-bundle	11m 1s
Test Node 14 on windows-latest	14m 50s
Test Node 14 on ubuntu-latest	12m 57s
lint	57s
baselines	8m 50s
package-size	1m 26s
smoke	59s
Test Node 24 on windows-latest	10m 9s
required	3s
	1h 25m 22s

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