cli: flag-driven interface (short+long); -u maximal blip-proof harvest

Replace the today/date/show/compare/update subcommands with a single
flag-driven interface like the Python predecessor: lectio [DATE]
-a/-b/-c/-r/-w/-R/-o/-l/-g/-u, short and long names bound to the same
variable, DATE (YYYY-MM-DD) extracted from any position in the args.

Fix Harvest's "-u" to walk to the true maximal horizon without a
transient network error being mistaken for it: a fetch failure now
retries a few times with backoff and, if it still fails, is reported
as an error (after saving partial progress via writeSigla), while a
Parse failure (the real unpublished-horizon placeholder) still stops
cleanly with a nil error.

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