Add coding agent instructions: refuse PRs unless maintenance mode is acknowledged #35540

Job	Run time
baselines	9m 46s
browser-integration	1m 45s
coverage	10m 19s
format	24s
package-size	1m 24s
lint	1m 0s
typecheck	44s
knip	36s
Test Node 14 on ubuntu-latest	12m 11s
misc	27s
smoke	58s
Test Node 24 on windows-latest	10m 25s
Test Node 24 on ubuntu-latest	8m 31s
Test Node 14 on windows-latest	15m 1s
self-check	56s
Test Node lts/* on ubuntu-latest with --no-bundle	10m 51s
required	2s
	1h 25m 20s

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