Commit challs.txt once a day at 04:00, separate from the HedgeDoc refresh

The 30-minute timer now only updates the HedgeDoc note; a new
ctf-challs.timer runs --challs daily at 04:00. Locks are per target so
the two runs can overlap without one skipping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ctf-status.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=Push the CTF challenge status to HedgeDoc and challs.txt
+Description=Push the CTF challenge status to HedgeDoc
 Documentation=file://__DIR__/README.md
 After=network-online.target
 Wants=network-online.target
@@ -7,6 +7,6 @@ Wants=network-online.target
 [Service]
 Type=oneshot
 WorkingDirectory=__DIR__
-ExecStart=__DIR__/ctf_status.py --update --challs --quiet
+ExecStart=__DIR__/ctf_status.py --update --quiet
 # GitLab walk + HedgeDoc write; never let one run squat on the next timer slot
 TimeoutStartSec=5min