feat: run profiles headlessly from cron or a systemd timer

[KiryuuLight]

lazyrsync run NAME printed the rsync command it would run and exited — the CLI said "(execution engine not wired yet)". Nothing existed for cron or a systemd timer to call. That bit hardest on Snapshot tasks, whose numbered destination and --link-dest chain are computed at run time, so the command differs every run and can't be written into a crontab by hand.

run PROFILE now executes every task in sequence; run PROFILE/TASK runs one by its id from list.

[1/3] ✔ Documents  1.2s
[2/3] ✗ Photos     exit 23  photos-3f2a
[3/3] ✔ Music      0.3s

3 tasks: 2 ok, 1 failed

Exit codes, --yes for destructive tasks, --verbose, and cron/systemd examples are in the README.

fmt, clippy, 96 tests clean.