docs(recovery): the undo runbook — generations, journals, snapper, nixos-enter (item 6d)

Slice (d) of the docs review: docs/RECOVERY.md tells the recovery
story that existed only in pieces, ordered by symptom — bad theme/HM
change (generations, apply-known-good, git-revert the flake), desktop
won't start (TTY, greetd + user journals, VM-GL and preactivate-log
pointers), broken system change (older systemd-boot generation, then
make it stick), lost files (btrfs-assistant / nomarchy-snapshots), and
the ISO + nixos-enter last resort. Commands checked against the tree;
linked from the README file tree and §5.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -76,6 +76,7 @@ Flat on purpose. Two module trees, one options file each, no hidden layers.
 ├── templates/downstream/          # `nix flake init -t` starter for users
 ├── docs/TESTING.md                # how to verify changes (incl. AI-agent rules)
 ├── docs/OVERRIDES.md              # how downstream users override defaults
+├── docs/RECOVERY.md               # runbook: broken theme/desktop/boot → undo it
 ├── docs/ROADMAP.md                # design/decision records + shipped-fixes log
 ├── agent/                         # autonomous-agent loop: protocol (LOOP.md),
 │                                  #   prioritized BACKLOG, journal, memory
@@ -361,6 +362,10 @@ sys-rebuild                            # rebuild the system, current lock (no up
 home-update                            # rebuild just the desktop layer
 ```
 
+Something broke anyway? Every rebuild is a generation and (on BTRFS)
+every hour is a snapshot — the undo story, from a bad theme to a
+machine that won't boot, is **[docs/RECOVERY.md](docs/RECOVERY.md)**.
+
 Keybinds: `SUPER+Return` terminal · `SUPER+D` launcher · `SUPER+T` theme
 picker · `SUPER+SHIFT+T` next wallpaper · `SUPER+X` power menu ·
 `SUPER+E` file manager (yazi) · `SUPER+N` notifications · `SUPER+CTRL+V`