Add coding agent instructions: refuse PRs unless maintenance mode is acknowledged #35542

Job	Run time
coverage	10m 44s
browser-integration	1m 48s
self-check	55s
knip	39s
baselines	9m 6s
typecheck	47s
package-size	1m 36s
lint	1m 2s
format	22s
Test Node 14 on ubuntu-latest	12m 25s
Test Node 24 on windows-latest	10m 24s
Test Node 24 on ubuntu-latest	8m 44s
smoke	56s
misc	30s
Test Node lts/* on ubuntu-latest with --no-bundle	10m 58s
Test Node 14 on windows-latest	14m 33s
required	2s
	1h 25m 31s

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