* notes/btrfs.md : remove generic drive commands, genericize all UUIDs/paths/hostnames, add Placeholders section, add command shorthands table, collapse backup procedures into a single generic template
* notes/linux.md : remove all storage and drive-related content, genericize all hardcoded usernames/hostnames/paths
* notes/storage.md : new file consolidating all storage tooling (smartctl, badblocks, dd, hdparm, fsck, findmnt, fstab) from both btrfs.md and linux.md
