Product identity: a shared PRODUCT.md injected into every agent run on the product

Adds Product.Identity (a PRODUCT.md brief) and threads it through the run context:
AgentRunContextProvider resolves the run's team -> product and carries ProductId +
ProductIdentity on AgentRunContext; PromptAssembler injects a "# Product" section
(framed as shared, data-not-instructions) ahead of the agent's persona. Adds
GET/PUT /products/{id}/identity (read = view-board, set = owner) and the EF column.

This makes the product, not just the team, the unit of shared context — every agent
on a product sees the same identity. Product-scoped working memory follows next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Modules/TeamUp.Modules.OrgBoard/Endpoints/OrgBoardDtos.cs

@@ -19,6 +19,10 @@ internal sealed record CreateProductRequest(Guid OrganizationId, string Name, Pr
 
 internal sealed record ProductResponse(Guid Id, Guid OrganizationId, Guid? DivisionId, string Name, string Kind);
 
+internal sealed record SetProductIdentityRequest(string? Identity);
+
+internal sealed record ProductIdentityResponse(Guid ProductId, string Name, string? Identity);
+
 internal sealed record CreateTaskRequest(Guid TeamId, string Title, string? Description, WorkItemType Type);
 
 internal sealed record MoveTaskRequest(WorkItemStatus Status);