An expired session exits 3 with one line saying to run quak login (closes #164)

A 401 from the server that ends a command reaches run in src/cli-run.ts as
an ApiError; run prints one line saying to run "quak login" and exits 3.
quak backup meets it on its first refresh, before it touches any file. For
the commands that load the saved session, a missing or corrupt session file
keeps its message and also exits 3.

Judgement call: quak logout is unchanged; it handles its own errors and
deletes the session file whatever the server answers.
Judgement call: quak backup still prints its two progress lines before the
error line.

Model: opus-5-5

src/cli-run.ts

@@ -1,12 +1,15 @@
 // Runs one CLI command for `bin/quak.ts` and exits with its code.
 
 import type { Writable } from "node:stream";
+import { ApiError } from "./api/client.js";
 
 // Run a command and exit with its code once stdout/stderr have drained.
 // Exiting before the drain can truncate piped output, and the library can keep
 // the event loop alive after a command returns, so a plain return could hang.
 // An error the command throws is printed as one `quak: MESSAGE` line, without
-// the stack trace, and exits 1.
+// the stack trace, and exits 1. A 401 from the server means it no longer
+// accepts the saved session: that prints one line saying to log in again and
+// exits 3, as a missing or corrupt session file does.
 export const run = async (
     command: Promise<number>,
     stdout: Writable,
@@ -17,10 +20,17 @@ export const run = async (
     try {
         code = await command;
     } catch (err) {
-        stderr.write(
-            `quak: ${err instanceof Error ? err.message : String(err)}\n`,
-        );
-        code = 1;
+        if (err instanceof ApiError && err.status === 401) {
+            stderr.write(
+                `quak: the saved session is no longer valid; run "quak login"\n`,
+            );
+            code = 3;
+        } else {
+            stderr.write(
+                `quak: ${err instanceof Error ? err.message : String(err)}\n`,
+            );
+            code = 1;
+        }
     }
     const pending = [stdout, stderr].filter((s) => s.writableLength > 0);
     if (pending.length === 0) {