refactor(langgraph): unify agent() return type as LangGraphAgent #448

Job	Run time
Library — lint / test / build	1m 23s
Cockpit — representative capability smoke	1m 13s
Cockpit — deploy smoke dry-run	1m 13s
Cockpit — build all examples	4m 52s
Cockpit — secret-gated integration	1m 8s
Website — lint / build	1m 48s
Website — e2e	1m 55s
Chat Agent — smoke	1m 13s
Cockpit — e2e	1m 56s
Cockpit — build / test	1m 25s
Production smoke	0s
Deploy → Vercel	0s
	18m 6s

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