refactor: rename the modes to say what they actually do

`full` was the misleading one: it is the abort-LIMITED run, the one that
deliberately stops enumerating a profile as soon as it reaches content already
held. Calling it "full" invited exactly the wrong assumption about coverage.
And `sweep` gave no hint that it was the exhaustive one.

    full   -> profiles     every surface, --abort 50, ~40-60 requests
    sweep  -> full-sweep   every surface, no abort,   ~420 requests

The old names now exit 2 with a pointer to the new one rather than a bare
"unknown mode", since muscle memory and any stray crontab will still use them.

full-sweep's description now says what it costs. At ~420 requests it is the
same order as the run that preceded the 2026-08-21 scraping warning, spent to
catch a handful of retroactively edited posts, so the docs suggest running it
by hand when you mean to rather than leaving it on a timer. Its cadence was
never actually agreed.

Units renamed to match and re-verified with systemd-analyze; the old ones are
removed from the host. All three timers remain disabled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UXfdJu7QhSJLr47K7koTDF

scripts/systemd/gdl-sync@full-sweep.timer

@@ -0,0 +1,18 @@
+[Unit]
+Description=Full Instagram archive sweep, no abort (~420 requests)
+# The only run that enumerates every profile to the end, and so the only one
+# that notices carousels edited after we archived them (test case 15).
+#
+# It is also by far the most expensive thing here: ~420 requests to
+# instagram.com, the same order as the run that preceded the 2026-08-21
+# scraping warning, spent to catch a handful of retroactively edited posts.
+# Consider running it by hand when you mean to, rather than on a timer.
+
+[Timer]
+# Month names are not valid in OnCalendar's date field -- numeric only.
+OnCalendar=*-01,04,07,10-07 04:00:00
+RandomizedDelaySec=45m
+Persistent=true
+
+[Install]
+WantedBy=timers.target