M4: the assembler — assemble → model → parse (Increment 2)

SharedKernel contracts (so Assembler stays decoupled): IAgentRunContextProvider (agent +
task) and ISkillCatalog (skill prompts by key). Implemented by OrgBoard (AgentRunContextProvider)
and Skills (SkillCatalog).

Assembler:
- PromptAssembler builds house-style + identity + the agent's skill bodies + the task, and
  derives the primary action + risk from the agent's first skill. RAG/working-memory join at M6.
- AgentRunExecutor (real): resolve context + skills → assemble → resolve BYOK config (with
  fallback) → call IModelClient → parse into action + risk → capture all on the AgentRun.

Verified: build green; ArchitectureTests 8/8; IntegrationTests 29/29 — incl. the M4 acceptance:
assigning a Spec task to Aria (PO, gated, stub BYOK) yields a Completed run with the assembled
prompt (skill body + task title), action "write-spec", risk "Draft", and model output. Nothing
executes — the gate is M5.

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

src/Shared/TeamUp.SharedKernel/Ai/ISkillCatalog.cs

@@ -0,0 +1,18 @@
+namespace TeamUp.SharedKernel.Ai;
+
+/// <summary>A skill's prompt body + its primary risk-tagged action, for prompt assembly.</summary>
+public sealed record SkillPrompt(
+    string Key,
+    string Name,
+    string Body,
+    string PrimaryAction,
+    string PrimaryActionRisk,
+    IReadOnlyList<string> Roles);
+
+/// <summary>Resolves skill prompts by key (latest version). Implemented by the Skills module.</summary>
+public interface ISkillCatalog
+{
+    Task<IReadOnlyList<SkillPrompt>> GetByKeysAsync(
+        IReadOnlyCollection<string> keys,
+        CancellationToken cancellationToken = default);
+}