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

A 401 from the server reaches run in src/cli-run.ts as the ApiError the
refresh threw, so run recognises it there, prints one line saying to run
"quak login" and exits 3. A missing or corrupt session file keeps its message
and also exits 3. quak backup meets the 401 on its first refresh, before it
touches any file. Every other error still exits 1.

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

TODO.md

@@ -25,6 +25,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-05: A command whose saved session the server no longer accepts
+  (HTTP 401) prints one line,
+  `quak: the saved session is no longer valid; run "quak login"`, and exits 3
+  (issue 164). A missing or corrupt session file keeps its message and now also
+  exits 3, so a cron job can tell that the user must log in again. `run` in
+  `src/cli-run.ts` recognises the 401, which reaches it unchanged from the
+  refresh, so `quak backup` stops there before it touches any file. Every other
+  error still exits 1.
+
 - 2026-10-03: The download-albums example test no longer fails on a disk with
   under 50 GiB free (issue 160). It opens its libraries with
   `freeBelowBytes: 0`, so the free space of the disk it runs on cannot shrink