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