Run the timers every 5 minutes, with overlap and failure guards

A push reaching the mirror should take minutes, not up to six hours. All four
units now fire every 5 minutes, staggered a minute apart. Three guards make
that cadence safe:

- Scheduled runs take the release lock NON-BLOCKING (try_release_lock) and
  skip the tick when a build is running. Blocking would stack one stalled
  process per tick behind a long build and stampede when it finished. Manual
  commands still wait, as an operator expects.
- check-versions takes the lock too, and now owns its git pull (--pull, passed
  by the unit) instead of an ExecStartPre: at this cadence an unlocked pull
  would swap PKGBUILDs out from under a running build.
- A failed release records .build-failed-<channel> and backs off
  exponentially (10m, 20m, 40m … capped at 6h) rather than rebuilding the same
  broken tree every 5 minutes. Any new commit clears the backoff, since a push
  is the most likely fix.

Idle ticks exit without output so the journal keeps showing the runs that
matter, and bin/repo timers reports backoff state — a paused channel is
otherwise indistinguishable from an idle one.

systemd/omarchy-auto-release-stable.timer

@@ -1,10 +1,10 @@
 [Unit]
-Description=Auto-release stable mirror every 6 hours
+Description=Release queued stable builds every 5 minutes
 
 [Timer]
-# Run every 6 hours at :00 (after version check at :30)
-OnCalendar=*-*-* 01,07,13,19:00:00 America/New_York
+OnCalendar=*:3/5
 Persistent=true
+RandomizedDelaySec=20
 
 [Install]
 WantedBy=timers.target