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/README.md

@@ -88,6 +88,20 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 |-----------|-------------|
 | `mac-system-theme.ts` | Syncs pi theme with macOS dark/light mode |
 
+### Messages & Communication
+
+| Extension | Description |
+|-----------|-------------|
+| `message-renderer.ts` | Custom message rendering with colors and expandable details via `registerMessageRenderer` |
+| `event-bus.ts` | Inter-extension communication via `pi.events` |
+
+### Session Metadata
+
+| Extension | Description |
+|-----------|-------------|
+| `session-name.ts` | Name sessions for the session selector via `setSessionName` |
+| `bookmark.ts` | Bookmark entries with labels for `/tree` navigation via `setLabel` |
+
 ### Custom Providers
 
 | Extension | Description |

packages/coding-agent/examples/extensions/bookmark.ts

@@ -0,0 +1,50 @@
+/**
+ * Entry bookmarking example.
+ *
+ * Shows setLabel to mark entries with labels for easy navigation in /tree.
+ * Labels appear in the tree view and help you find important points.
+ *
+ * Usage: /bookmark [label] - bookmark the last assistant message
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("bookmark", {
+		description: "Bookmark last message (usage: /bookmark [label])",
+		handler: async (args, ctx) => {
+			const label = args.trim() || `bookmark-${Date.now()}`;
+
+			// Find the last assistant message entry
+			const entries = ctx.sessionManager.getEntries();
+			for (let i = entries.length - 1; i >= 0; i--) {
+				const entry = entries[i];
+				if (entry.type === "message" && entry.message.role === "assistant") {
+					pi.setLabel(entry.id, label);
+					ctx.ui.notify(`Bookmarked as: ${label}`, "info");
+					return;
+				}
+			}
+
+			ctx.ui.notify("No assistant message to bookmark", "warning");
+		},
+	});
+
+	// Remove bookmark
+	pi.registerCommand("unbookmark", {
+		description: "Remove bookmark from last labeled entry",
+		handler: async (_args, ctx) => {
+			const entries = ctx.sessionManager.getEntries();
+			for (let i = entries.length - 1; i >= 0; i--) {
+				const entry = entries[i];
+				const label = ctx.sessionManager.getLabel(entry.id);
+				if (label) {
+					pi.setLabel(entry.id, undefined);
+					ctx.ui.notify(`Removed bookmark: ${label}`, "info");
+					return;
+				}
+			}
+			ctx.ui.notify("No bookmarked entry found", "warning");
+		},
+	});
+}

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",
+		});
+	});
+}

packages/coding-agent/examples/extensions/message-renderer.ts

@@ -0,0 +1,59 @@
+/**
+ * Custom message rendering example.
+ *
+ * Shows how to use registerMessageRenderer to control how custom messages
+ * appear in the TUI, with colors, formatting, and expandable details.
+ *
+ * Usage: /status [message] - sends a status message with custom rendering
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import { Box, Text } from "@mariozechner/pi-tui";
+
+export default function (pi: ExtensionAPI) {
+	// Register custom renderer for "status-update" messages
+	pi.registerMessageRenderer("status-update", (message, { expanded }, theme) => {
+		const details = message.details as { level: string; timestamp: number } | undefined;
+		const level = details?.level ?? "info";
+
+		// Color based on level
+		const color = level === "error" ? "error" : level === "warn" ? "warning" : "success";
+		const prefix = theme.fg(color, `[${level.toUpperCase()}]`);
+
+		let text = `${prefix} ${message.content}`;
+
+		// Show timestamp when expanded
+		if (expanded && details?.timestamp) {
+			const time = new Date(details.timestamp).toLocaleTimeString();
+			text += `\n${theme.fg("dim", `  at ${time}`)}`;
+		}
+
+		// Use Box with customMessageBg for consistent styling
+		const box = new Box(1, 1, (t) => theme.bg("customMessageBg", t));
+		box.addChild(new Text(text, 0, 0));
+		return box;
+	});
+
+	// Command to send status messages
+	pi.registerCommand("status", {
+		description: "Send a status message (usage: /status [warn|error] message)",
+		handler: async (args, _ctx) => {
+			const parts = args.trim().split(/\s+/);
+			let level = "info";
+			let content = args.trim();
+
+			// Check for level prefix
+			if (parts[0] === "warn" || parts[0] === "error") {
+				level = parts[0];
+				content = parts.slice(1).join(" ") || "Status update";
+			}
+
+			pi.sendMessage({
+				customType: "status-update",
+				content,
+				display: true,
+				details: { level, timestamp: Date.now() },
+			});
+		},
+	});
+}

packages/coding-agent/examples/extensions/session-name.ts

@@ -0,0 +1,27 @@
+/**
+ * Session naming example.
+ *
+ * Shows setSessionName/getSessionName to give sessions friendly names
+ * that appear in the session selector instead of the first message.
+ *
+ * Usage: /session-name [name] - set or show session name
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("session-name", {
+		description: "Set or show session name (usage: /session-name [new name])",
+		handler: async (args, ctx) => {
+			const name = args.trim();
+
+			if (name) {
+				pi.setSessionName(name);
+				ctx.ui.notify(`Session named: ${name}`, "info");
+			} else {
+				const current = pi.getSessionName();
+				ctx.ui.notify(current ? `Session: ${current}` : "No session name set", "info");
+			}
+		},
+	});
+}