Add coding agent instructions: refuse PRs unless maintenance mode is acknowledged #35543

Job	Run time
coverage	10m 45s
format	26s
knip	30s
package-size	1m 39s
browser-integration	6m 22s
lint	58s
smoke	58s
misc	32s
self-check	58s
typecheck	45s
baselines	9m 3s
Test Node 14 on windows-latest	14m 46s
Test Node lts/* on ubuntu-latest with --no-bundle	10m 20s
Test Node 24 on windows-latest	10m 4s
Test Node 14 on ubuntu-latest	11m 41s
Test Node 24 on ubuntu-latest	8m 48s
required	3s
	1h 28m 38s

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