chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/coding-agent/examples/extensions/system-prompt-header.ts

@@ -0,0 +1,17 @@
+/**
+ * Displays a status widget showing the system prompt length.
+ *
+ * Demonstrates ctx.getSystemPrompt() for accessing the effective system prompt.
+ */
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("agent_start", (_event, ctx) => {
+		const prompt = ctx.getSystemPrompt();
+		ctx.ui.setStatus("system-prompt", `System: ${prompt.length} chars`);
+	});
+
+	pi.on("session_shutdown", (_event, ctx) => {
+		ctx.ui.setStatus("system-prompt", undefined);
+	});
+}