docs(examples): expand hello_seller.py to full 9-method sales-non-gua… #1200

Job	Run time
Downstream import smoke (representative consumer symbols)	25s
AdCP storyboard runner — v3 reference seller (translator)	1m 19s
AdCP storyboard runner — examples/seller_agent.py	1m 35s
v3 reference seller — pytest (respx-mocked upstream)	36s
AdCP storyboard runner — examples/multi_platform_seller (PlatformRouter)	1m 25s
Postgres conformance tests (Postgres 16)	1m 1s
Test Python 3.12	3m 27s
Validate schemas are up-to-date	3m 22s
Test Python 3.11	3m 19s
Test Python 3.10	3m 41s
Test Python 3.13	3m 28s
Validate conventional commit format	0s
	23m 38s

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