Spec: add sync_governance response schema

[bokelley]

Summary

The sync_governance task is defined in the AdCP spec and used in the media_buy_seller storyboard, but there is no corresponding response schema (sync-governance-response.json) in the spec repo. This means @adcp/client cannot register a Zod schema for response validation.

Impact

Storyboard steps with check: response_schema for sync_governance fail with:

❌ Response matches sync-governance-response.json schema
   No schema registered for task "sync_governance"

Expected

Add docs/schemas/account/sync-governance-response.json defining the response shape. Based on usage in the storyboard YAML, the response should acknowledge the registered governance agents (confirmation of storage).

Found by

adcp storyboard run test-mcp media_buy_seller in @adcp/client PR #424.