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-profiles.sh

@@ -3,4 +3,7 @@
 # does this), ~5 min. Covers everything but is no substitute for stories.sh --
 # a story posted and expired between two runs of this is simply gone.
 set -eu
+# healthchecks.io check "instagram-archiver-posts". Override or unset (e.g.
+# HC_URL= ./run-profiles.sh) to run without pinging it.
+export HC_URL="${HC_URL:-https://hc-ping.com/3f40abd8-e9bd-43af-a105-c583efaad88c}"
 exec "$(dirname "$0")/gdl-cron.sh" profiles "$@"