refactor: rename ChatAgent → Agent across the contract surface #413

Job	Run time
Library — lint / test / build	1m 46s
Website — lint / build	1m 54s
Cockpit — build all examples	4m 36s
Cockpit — representative capability smoke	1m 20s
Chat Agent — smoke	1m 31s
Cockpit — e2e	2m 11s
Cockpit — secret-gated integration	1m 15s
Cockpit — build / test	1m 46s
Cockpit — deploy smoke dry-run	1m 24s
Website — e2e	2m 19s
Deploy → Vercel	0s
Production smoke	0s
	20m 2s

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