v0.3.0 — 5 new scripts, reorganized categories, standardized headers

New scripts:
- monitoring/system-health.sh   interactive OK/WARN/CRIT health overview
- monitoring/memory-alert.sh    RAM + swap monitoring with top processes
- monitoring/pending-updates.sh apt/dnf/yum security update checker
- security/open-ports-audit.sh  listening port audit with process names
- security/user-audit.sh        user accounts, sudo access, SSH keys audit

Breaking change — category restructure (15 folders → 4):
- monitoring/  disk-alert, memory-alert, pending-updates, service-watchdog,
               ssl-expiry-check, system-health, system-info
- security/    cron-audit, fail2ban-report, open-ports-audit, ssh-audit,
               user-audit
- maintenance/ backup, log-rotate
- benchmarks/  bench

All scripts: standardized headers (DESCRIPTION / USAGE / OPTIONS /
EXIT CODES / CRON EXAMPLE / REQUIREMENTS) and --help flag via a
self-extracting usage() function.