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/Persistence/Migrations/OrgBoardDbContextModelSnapshot.cs

@@ -225,6 +225,9 @@ namespace TeamUp.Modules.OrgBoard.Persistence.Migrations
                     b.Property<Guid?>("DivisionId")
                         .HasColumnType("uuid");
 
+                    b.Property<string>("Identity")
+                        .HasColumnType("text");
+
                     b.Property<string>("Kind")
                         .IsRequired()
                         .HasMaxLength(16)