feat(signing): key_origins check + brand.json chain error codes (#350 stage 4) #1620

Job	Run time
AdCP storyboard runner — v3 reference seller (translator)	2m 1s
Test Python 3.10	4m 23s
AdCP storyboard runner — examples/seller_agent.py	1m 19s
Test Python 3.12	4m 10s
Postgres conformance tests (Postgres 16)	1m 6s
AdCP storyboard runner — sales-proposal-mode (proposal_finalize)	1m 9s
Test Python 3.13	4m 12s
Downstream import smoke (representative consumer symbols)	30s
Validate conventional commit format	10s
v3 reference seller — pytest (respx-mocked upstream)	40s
Test Python 3.11	4m 25s
Validate schemas are up-to-date	3m 25s
AdCP storyboard runner — examples/multi_platform_seller (PlatformRouter)	1m 40s
	29m 10s

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