Add file-trigger hook example

packages/coding-agent/examples/hooks/file-trigger.ts

@@ -0,0 +1,36 @@
+/**
+ * File Trigger Hook
+ *
+ * Watches a trigger file and injects its contents into the conversation.
+ * Useful for external systems to send messages to the agent.
+ *
+ * Usage:
+ *   echo "Run the tests" > /tmp/agent-trigger.txt
+ */
+
+import * as fs from "node:fs";
+import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function (pi: HookAPI) {
+	pi.on("session", async (event, ctx) => {
+		if (event.reason !== "start") return;
+
+		const triggerFile = "/tmp/agent-trigger.txt";
+
+		fs.watch(triggerFile, () => {
+			try {
+				const content = fs.readFileSync(triggerFile, "utf-8").trim();
+				if (content) {
+					pi.send(`External trigger: ${content}`);
+					fs.writeFileSync(triggerFile, ""); // Clear after reading
+				}
+			} catch {
+				// File might not exist yet
+			}
+		});
+
+		if (ctx.hasUI) {
+			ctx.ui.notify(`Watching ${triggerFile}`, "info");
+		}
+	});
+}