fix(coding-agent): add pnpm self-update prune hint (#6279)

Adds a pnpm-specific self-update recovery hint when pi update fails during self-update.
Fixes #6215.

packages/coding-agent/src/package-manager-cli.ts

@@ -372,6 +372,11 @@ function printSelfUpdateFallback(command: SelfUpdateCommand): void {
 	console.error(chalk.dim(`If this keeps failing, run this command yourself: ${command.display}`));
 }
 
+function printPnpmSelfUpdateMetadataHint(): void {
+	console.error(chalk.yellow("If pnpm reports missing package versions, its cached registry metadata may be stale."));
+	console.error(chalk.yellow(`Run \`pnpm store prune\` and retry \`${APP_NAME} update --self\`.`));
+}
+
 function printSelfUpdateNote(note: string): void {
 	const trimmedNote = note.trim();
 	if (!trimmedNote) {
@@ -753,6 +758,9 @@ export async function handlePackageCommand(
 					} catch (error: unknown) {
 						const message = error instanceof Error ? error.message : "Unknown package command error";
 						console.error(chalk.red(`Error: ${message}`));
+						if (installMethod === "pnpm") {
+							printPnpmSelfUpdateMetadataHint();
+						}
 						printSelfUpdateFallback(selfUpdateCommand);
 						process.exitCode = 1;
 						return true;