feat(menu): System › Rollback — desktop generations picker (item 9b, item 9 done)

Undo is one menu away: the new rollback flow lists the last ten Home
Manager generations (the theme/config history is already generations,
newest marked current) and activates the picked one in a terminal —
HM's supported rollback, reversible by re-picking a newer one, so no
typed-yes gate. System-level undo deliberately links out rather than
reimplementing destructive flows in rofi: the Snapshots row opens the
existing gated flow, and 'boot an older generation (how)' fires an
instruction notification pointing at docs/RECOVERY.md §3 (which now
cross-references the menu path).

V1: template-home built, generated menu bash -n clean, row rendering
and pick-parsing verified against fixture generations output. V3
exercise queued.

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

agent/HARDWARE-QUEUE.md

@@ -86,6 +86,13 @@ QA machine), the **T14s** (webcam case).
       `sudo nomarchy-snapshots`, restore a single file (`undochange`)
       and walk a root-config rollback up to (or through) the
       typed-`yes` gate.
+- [ ] **Rollback menu (item 9b)** — after a couple of theme changes,
+      menu › System › Rollback: recent desktop generations listed
+      (newest marked current); picking an older one opens a terminal,
+      activates it, and the theme visibly reverts; picking the newer one
+      again rolls forward. The two System rows: Snapshots opens the
+      snapshot flow, "boot an older generation (how)" fires an
+      instruction notification.
 - [ ] **Open-a-file smoke (viewers + mime defaults, item 8)** — after
       `home-update`: from yazi/Thunar, open a PDF (→ zathura, themed to
       the palette), an image (→ imv, NOT GIMP), a video (→ mpv);