fix(chat): revert a2ui assistant ID to c-a2ui to match deployment #365

Job	Run time
Cockpit — build / test	1m 34s
Chat Agent — smoke	1m 15s
Website — lint / build	1m 37s
Cockpit — build all examples	4m 58s
Library — lint / test / build	1m 24s
Cockpit — e2e	1m 44s
Cockpit — representative capability smoke	1m 13s
Website — e2e	2m 1s
Cockpit — secret-gated integration	1m 15s
Cockpit — deploy smoke dry-run	59s
Deploy → Vercel	0s
Production smoke	0s
	18m 0s

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