feat(FN-742): add describeModel helper and log model in agent creation sites

- Add describeModel() helper in pi.ts to format provider/model info for logging
- Log resolved model details in executor, reviewer, and triage agent creation
- Update executor and reviewer to call describeModel before session start
- Add unit tests for describeModel covering all input combinations
- Fix test mocks to account for new describeModel dependency

packages/engine/src/executor.test.ts

@@ -4,6 +4,7 @@ import { AgentSemaphore } from "./concurrency.js";
 // Mock external dependencies
 vi.mock("./pi.js", () => ({
   createKbAgent: vi.fn(),
+  describeModel: vi.fn().mockReturnValue("mock-provider/mock-model"),
 }));
 vi.mock("./reviewer.js", () => ({
   reviewStep: vi.fn(),