docs(coding-agent): improve extensions.md, add missing examples

- Add cross-links to session.md, keybindings.md, themes.md
- Document truncateLine, highlightCode, getLanguageFromPath utilities
- Add examples for previously undocumented APIs:
  - message-renderer.ts: registerMessageRenderer with Box styling
  - session-name.ts: setSessionName/getSessionName
  - event-bus.ts: pi.events for inter-extension communication
  - bookmark.ts: setLabel for entry bookmarking
- Update examples table in extensions.md and README.md

packages/coding-agent/examples/extensions/event-bus.ts

@@ -0,0 +1,43 @@
+/**
+ * Inter-extension event bus example.
+ *
+ * Shows pi.events for communication between extensions. One extension
+ * can emit events that other extensions listen to.
+ *
+ * Usage: /emit [event-name] [data] - emit an event on the bus
+ */
+
+import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	// Store ctx for use in event handler
+	let currentCtx: ExtensionContext | undefined;
+
+	pi.on("session_start", async (_event, ctx) => {
+		currentCtx = ctx;
+	});
+
+	// Listen for events from other extensions
+	pi.events.on("my:notification", (data) => {
+		const { message, from } = data as { message: string; from: string };
+		currentCtx?.ui.notify(`Event from ${from}: ${message}`, "info");
+	});
+
+	// Command to emit events (emits "my:notification" which the listener above receives)
+	pi.registerCommand("emit", {
+		description: "Emit my:notification event (usage: /emit message)",
+		handler: async (args, _ctx) => {
+			const message = args.trim() || "hello";
+			pi.events.emit("my:notification", { message, from: "/emit command" });
+			// Listener above will show the notification
+		},
+	});
+
+	// Example: emit on session start
+	pi.on("session_start", async () => {
+		pi.events.emit("my:notification", {
+			message: "Session started",
+			from: "event-bus-example",
+		});
+	});
+}