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>

agent/BACKLOG.md

@@ -30,14 +30,14 @@ surface vs tables, catches stale rows too); slice (b) drift pass over
 docs/OVERRIDES.md, docs/TESTING.md, templates/downstream/README.md —
 every factual claim spot-checked against the tree, all held; fixed the
 two gaps (OVERRIDES now routes to `nomarchy.monitors`; TESTING §1 lists
-the shell-syntax check its CI § claimed). Remaining slices:
+the shell-syntax check its CI § claimed); slice (d) recovery runbook —
+docs/RECOVERY.md (theme/HM generations → greetd journals → boot an older
+generation → snapper restore/rollback → nixos-enter), linked from the
+README tree + §5. Remaining:
 - **(c) install/first-run story** read end to end (README install §§ +
-  template README as one narrative; fix the seams).
-- **(d) recovery runbook** — short doc: desktop won't start → boot an
-  older generation / `nomarchy-snapshots` / greetd journal. The pieces
-  exist, the story isn't written.
-Site-vs-markdown is a Decision (below). **Verify:** V0 per slice; (b)/(c)
-against the live tree, not memory.
+  template README as one narrative; fix the seams) — the last slice.
+Site-vs-markdown is a Decision (below). **Verify:** V0; (c) against the
+live tree, not memory.
 
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