feat(coding-agent): add model_select extension hook

Fires when the model changes via /model command, model cycling (Ctrl+P),
or session restore. Includes source field and previousModel.

Add model-status.ts example extension demonstrating status bar updates.

closes #628

packages/coding-agent/examples/extensions/model-status.ts

@@ -0,0 +1,31 @@
+/**
+ * Model status extension - shows model changes in the status bar.
+ *
+ * Demonstrates the `model_select` hook which fires when the model changes
+ * via /model command, Ctrl+P cycling, or session restore.
+ *
+ * Usage: pi -e ./model-status.ts
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("model_select", async (event, ctx) => {
+		const { model, previousModel, source } = event;
+
+		// Format model identifiers
+		const next = `${model.provider}/${model.id}`;
+		const prev = previousModel ? `${previousModel.provider}/${previousModel.id}` : "none";
+
+		// Show notification on change
+		if (source !== "restore") {
+			ctx.ui.notify(`Model: ${next}`, "info");
+		}
+
+		// Update status bar with current model
+		ctx.ui.setStatus("model", `🤖 ${model.id}`);
+
+		// Log change details (visible in debug output)
+		console.log(`[model_select] ${prev} → ${next} (${source})`);
+	});
+}