docs: bring the README in line with what the scripts now do

The old README was a flat list of one-line descriptions, several of
which no longer described the script: log_rotate gained flags and no
longer purges by default, restore and process_monitor now confirm before
acting, rsync_magic changed a default, and disk_cleanup grew guards.

Rewritten around the conventions that are now consistent across the
collection -- --dry-run, a confirmation before anything destructive, an
unattended escape hatch, and a refusal rather than a guess when there is
no tty to ask on. That last one is the part worth knowing before putting
any of these in cron.

Groups the scripts by what they are for rather than listing them
alphabetically, and states the things a reader would otherwise have to
discover by reading source: that security_audit as an ordinary user
proves very little, that log_rotate is not a logrotate replacement and
why, that update_system is unattended on every branch, and that
restore's tar options matter because an archive picks its own ownership
and modes.

Also updated three script headers that had gained ASSUME_YES/FORCE
escapes without documenting them, so the headers and the README agree.

Every flag, environment variable and behaviour claimed here was checked
against the scripts rather than written from memory.

user_manage.sh

@@ -20,7 +20,9 @@
 #                addtogroup <user> <group>, lock <user>, unlock <user>,
 #                listusers, listgroups
 # Description: Automates user/group creation, deletion, and modifications.
-# Requires root privileges for most operations.
+# Requires root for everything except listusers/listgroups.
+# deluser removes the home directory and asks first; set ASSUME_YES=1 to
+# skip that prompt for unattended use.
 
 set -euo pipefail