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/pi.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from "vitest";
+import { describeModel } from "./pi.js";
+import type { AgentSession } from "@mariozechner/pi-coding-agent";
+
+describe("describeModel", () => {
+  it('returns "provider/modelId" when session has a model', () => {
+    const fakeSession = {
+      model: {
+        provider: "anthropic",
+        id: "claude-sonnet-4-5",
+        name: "Claude Sonnet",
+      },
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("anthropic/claude-sonnet-4-5");
+  });
+
+  it('returns "unknown model" when session model is undefined', () => {
+    const fakeSession = {
+      model: undefined,
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("unknown model");
+  });
+
+  it("handles different providers", () => {
+    const fakeSession = {
+      model: {
+        provider: "openai",
+        id: "gpt-4o",
+        name: "GPT-4o",
+      },
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("openai/gpt-4o");
+  });
+});