feat(desktop): viewers (zathura+imv) + xdg-mime defaults (item 8)

The complete-workstation gap: no PDF/image viewer shipped and nothing
set mime defaults, so 'open a photo' fell to GIMP. Two new default-on
toggles: nomarchy.viewers (programs.zathura — Stylix themes it, target
added to the explicit list — plus imv) and nomarchy.mime (mkDefault
xdg.mimeApps: pdf→zathura, image→imv, av→mpv, text→code, dir→thunar;
html/http(s)→firefox as inert entries that only activate when a
browser is installed — the ship-a-browser Decision stays open, now
smaller). Associations for absent apps are skipped by GIO, so the
whole thing degrades as the user curates the suite. Deviation from
the backlog text: viewers are a module toggle, not template package
lines — zathura's theming needs its HM module (the real-config
boundary of the toggle discipline).

V1: template-home generation asserted — mimeapps.list rendered, themed
zathurarc, binaries in home-path. V3 open-a-file smoke 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.
+- [ ] **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);
+      `xdg-open .` on a directory → Thunar. With no browser installed,
+      clicking a link must still fall through to *something sane* (the
+      firefox.desktop entries are inert); if you've uncommented a
+      browser, links go there.
 - [ ] **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