feat(decisioning): v6.0 DecisioningPlatform foundation (skeleton + design) #847

Job	Run time
PgReplayStore tests (Postgres 16)	1m 4s
Validate schemas are up-to-date	3m 5s
Test Python 3.12	3m 15s
Validate conventional commit format	10s
Test Python 3.11	3m 16s
Test Python 3.13	2m 42s
Test Python 3.10	3m 10s
AdCP storyboard runner — examples/seller_agent.py	43s
Downstream import smoke (representative consumer symbols)	28s
	17m 53s

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