fix(tui): update welcome banner model name on model_changed event

The welcome banner (WelcomeComponent) captures the session's active
model name at init time but never receives updates when the model
changes. Its setModel() method already existed but was never called
after construction.

This wires the model_changed agent session event to a new
#updateWelcomeModel() method in InteractiveMode, mirroring the
existing #updateWelcomeLspServers() pattern. The banner now reflects
the live model after:
- delayed config/modelRoles load (the startup race where an
  alphabetically-first provider is picked before config resolves)
- explicit /model switches
- retry-fallback model swaps

packages/coding-agent/test/modes/components/welcome.test.ts

@@ -62,3 +62,21 @@ describe("WelcomeComponent tips", () => {
 		expect(pickWeightedTip([], 0.5)).toBe("");
 	});
 });
+
+describe("WelcomeComponent model name", () => {
+	beforeAll(async () => {
+		await Settings.init({ inMemory: true });
+		await initTheme(false);
+	});
+
+	it("updates the rendered model name after setModel()", () => {
+		const welcome = new WelcomeComponent("1.0.0", "DeepSeek V4 Pro", "deepseek");
+		const before = welcome.render(100).join("\n");
+		expect(before).toContain("DeepSeek V4 Pro");
+
+		welcome.setModel("GLM-5.2", "zhipu-coding-plan");
+		const after = welcome.render(100).join("\n");
+		expect(after).toContain("GLM-5.2");
+		expect(after).not.toContain("DeepSeek V4 Pro");
+	});
+});