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/BACKLOG.md

@@ -22,16 +22,6 @@ next, in what order*.
 
 ## NOW
 
-### 4. btrfs-assistant segfault — attempt the nixpkgs override fix
-ROADMAP § Snapshot browse/restore. btrfs-assistant 2.2 crashes in
-`libbtrfsutil.so.1.4.0` (ABI mismatch, confirmed not a VM artifact); the
-`nomarchy-snapshots` fzf fallback ships. Try an override aligning its
-libbtrfsutil (or a version bump/pin); if upstream nixpkgs has since
-fixed it, just verify and re-point the menu. **Done when:** the GUI
-launches in the VM test, or the attempt is written up as infeasible
-(then re-check on each lock bump). **Verify:** V2 — extend the snapper
-VM check to launch the binary.
-
 ### 5. Keyboard layouts — cycle bind + summer-bar parity
 ROADMAP § Keyboard layouts, remaining bullets: (a) a multi-layout cycle
 bind (`hyprctl switchxkblayout` current-device, or xkb `grp:` option)