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/Shared/TeamUp.SharedKernel/Ai/IAgentRunContextProvider.cs

@@ -23,7 +23,9 @@ public sealed record AgentRunContext(
     string? TaskDescription,
     string TaskType,
     Guid TeamId,
-    Guid OrganizationId);
+    Guid OrganizationId,
+    Guid? ProductId = null,
+    string? ProductIdentity = null);
 
 /// <summary>Resolves the run context for a (seat, task) pair. Implemented by OrgBoard.</summary>
 public interface IAgentRunContextProvider