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/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() },
+			});
+		},
+	});
+}