feat: add healthchecks.io pings to run-stories.sh and run-profiles.sh

Start/finish pings wired through gdl-cron.sh via HC_URL, finish ping
carries the run's log as its body (visible in the dashboard, no SSH
needed to diagnose a failure). Exit code is the trailing URL segment
by convention, so the existing "stories: 0 to sync" -> exit 75 remap
already reaches the dashboard as a failure too.

Also fixes the actual bug that would have made this useless: the
routine chown-permission rsync warning (the archive share is never
owned by the ssh user) made gdl-sync.py return 1 on every clean run,
not just failing ones -- harmless when a human reads the log, but it
would have pinged every successful run as a failure. publish() now
only masks that one specific, verified-benign error shape; a real
transfer failure sharing the same exit code (23) still fails loudly.
6 new tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011qAds5qr7nZRq5R4yAuxUk

scripts/run-stories.sh

@@ -6,4 +6,7 @@
 # mode argument; override pacing/paths the same way gdl-cron.sh does, with
 # GDL_* env vars (see gdl-cron.sh's own comments for what's available).
 set -eu
+# healthchecks.io check "instagram-archiver-stories". Override or unset (e.g.
+# HC_URL= ./run-stories.sh) to run without pinging it.
+export HC_URL="${HC_URL:-https://hc-ping.com/9fe7347f-95be-47aa-a46b-affd27691554}"
 exec "$(dirname "$0")/gdl-cron.sh" stories "$@"