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