fix(snapshots): un-break the GUI — polkit agent + launcher, not an override

Re-diagnosis of the "btrfs-assistant 2.2 segfaults on launch" bug, and
the fix is not the planned nixpkgs override:

1. gdb: the crash is btrfs_util_subvolume_iterator_next() —
   libbtrfsutil's UNPRIVILEGED subvolume-iteration path in btrfs-progs
   6.17.1 (upstream-fixed after 6.17.1, kdave/btrfs-progs 886571653).
   Symbol versions were checked and match: not ABI/link drift.
2. VM A/B on real btrfs: as root it works (exit 0); as a user it
   segfaults (139). The pkexec launcher runs it as root — so the GUI
   path was fine all along, except…
3. …the distro shipped NO polkit authentication agent: every pkexec
   prompt in the session failed silently. That is the actual root
   cause of "the GUI doesn't open".

Shipped: hyprpolkitagent (Hyprland's Qt agent — the prompt is
Stylix-themed) via exec-once in hyprland.nix; `nomarchy-menu snapshot`
prefers btrfs-assistant-launcher again, with the nomarchy-snapshots
fzf flow kept as fallback (and for SSH); checks.snapshot-gui guards
the root path + offscreen GUI event loop on a real btrfs volume so a
lock bump can't silently regress it. No btrfs-progs patch: all our
flows are root-side; the unprivileged fix arrives with a lock bump.

Verified: V0 (flake check) + V2 (snapshot-gui check, executed green) +
V1 (HM generation builds; hyprland.conf carries the agent exec-once;
generated menu passes bash -n with the launcher wiring). Remaining V3
queued: first on-hardware themed polkit prompt.

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

agent/HARDWARE-QUEUE.md

@@ -25,9 +25,14 @@ QA machine), the **T14s** (webcam case).
       plug an external keyboard post-login, pick a layout in rofi, confirm
       it applies per-device only, persists in `settings.keyboard.devices`,
       and graduates into a `device{}` block on the next rebuild.
-- [ ] **Snapshots restore/rollback** — `nomarchy-menu snapshot` →
-      `nomarchy-snapshots`: browse/diff, restore a file (`undochange`),
-      and a root-config rollback, each behind the typed-`yes` gate.
+- [ ] **Snapshots restore/rollback** — `nomarchy-menu snapshot` now opens
+      the **btrfs-assistant GUI**: a *themed* polkit prompt must appear
+      (hyprpolkitagent — first on-hardware outing) and the GUI must open
+      as root. Also exercise the fzf fallback in a terminal
+      (`sudo nomarchy-snapshots`): browse/diff, restore a file
+      (`undochange`), and a root-config rollback behind the typed-`yes`
+      gate. Bonus check: any other pkexec flow now prompts instead of
+      silently failing.
 - [ ] **Update awareness** — with `nomarchy.updates.enable`, let the timer
       fire (or start the unit): indicator appears only when inputs are
       behind, notification only on count growth, click opens the upgrade