feat: Impermanence (system)

I had to disable initrd's systemd stuff.  I just /couldn't/ get a rollback
service working; believe me, not for a lack of effort!  I've been working on
this on-and-off for a month or two now.

README.org

@@ -11,10 +11,6 @@ A second try at NixOS, now that I have a better idea of what I'm doing. The effo
 
 * Inbox
 
-** Refactor: single file per feature
-
-Modules, but lightweight.
-
 * Principles
 
 ** User configuration
@@ -75,7 +71,17 @@ Where =default.nix= returns an attrset of form
 
 ** Impermanence and persistence
 
-- Persistent files to be linked into ~/~ go under ~/persist/root~
+I use impermanence to wipe most of my filesystem on boot.
+
+*** Boot process
+
+What follows is an overview of [[file:modules/nixos/impermanence/rollback.nix][modules/nixos/impermanence/rollback.nix]].
+
+On boot, ...
+
+- The existing subvolume root filesystem will be moved to a 'death row' directory, where it will live for about three days before deletion. Precisely, =«btrfs-filesystem»/«root-subvolume»= is moved to =«btrfs-filesystem»/old-roots/«timestamp»=. The brief grace period allows for easy recovery in the (very common) case where files are unintentionally deleted due to the user's silly human negligence.
+- A new, blank subvolume is created in place of the previous. Precisely, the subvolume =«btrfs-filesystem»/«root-subvolume»= is created.
+- Any subvolumes under =«btrfs-filesystem»/old-roots= older than three days are deleted.
 
 * Tasks
 
@@ -301,6 +307,8 @@ https://discourse.nixos.org/t/what-to-do-with-a-full-boot-partition/2049
       - If neither are available, use ~nix run nixpkgs#nvim~
         - If this fails, try ~nano~.
 
+- Support ~--pager~
+
 *** TODO =rage-edit=
 
 *** TODO ~forget-host HOST~