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

TODO.md

@@ -25,6 +25,16 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: When the server answers HTTP 401 and that ends a command that
+  loads the saved session, because the server no longer accepts its token, the
+  command prints one line,
+  `quak: the saved session is no longer valid; run "quak login"`, and exits 3
+  (issue 164). `quak backup` meets that 401 on the refresh it starts with,
+  before it touches any file. For the same commands, a missing or corrupt
+  session file keeps its message and also exits 3, so a cron job can tell that
+  the user must log in again. `quak logout` is unchanged. `run` in
+  `src/cli-run.ts` recognises the 401, which reaches it unchanged.
+
 - 2026-10-06: `quak backup` retries a failed request for longer than the other
   commands do (issue 165). `src/retry.ts` exports `UNATTENDED_RETRY_OPTIONS`
   beside the unchanged default: 10 attempts, a 1 s base delay and a 60 s cap, so