MCP tool-use execution loop for autonomous agent runs

Autonomous agents with MCP tools now run a bounded tool-use loop: the model may
call tools (executed via the gateway, results fed back) until it returns a final
answer. Gated/DraftOnly agents get the tool catalog as data but never auto-call —
a human-in-the-loop agent never autonomously reaches an external tool.

Extends IModelClient with tool definitions and a tool-use conversation, adds the
OpenAI-compatible tool serialization/parsing plus a deterministic "tooluse" stub
client, and records every tool call in the run trace.

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

src/Modules/TeamUp.Modules.Integrations/IntegrationsModule.cs

@@ -38,6 +38,7 @@ public sealed class IntegrationsModule : IModule
 
         // Model clients: a router over per-provider adapters.
         services.AddSingleton<StubModelClient>();
+        services.AddSingleton<ToolUseStubModelClient>();
         services.AddHttpClient<OpenAiCompatibleModelClient>();
         services.AddScoped<IModelClient, ModelClientRouter>();