fix(mime): text/plain falls through to Text Editor on live (#119)

A singleton default naming code.desktop left the live ISO with no text
handler — GIO skips missing .desktop files and offers nothing. Prefer
vscode when present, then org.gnome.TextEditor.desktop. Live also gains
mpv (video defaults had the same hole; free via the template pin).

checks.live-baseline-apps now requires every Default Applications key
to resolve to a present .desktop on live (HM + system path) and on the
default template install. Proved to fail on a vscode-only text/plain.

Verified: V2 (live-baseline-apps green + prove-to-fail); V0 flake check.

agent/HARDWARE-QUEUE.md

@@ -421,12 +421,11 @@ the **T14s** (webcam case).
       **LibreOffice** (Writer), **Text Editor**, **Amberol**, **Snapshot**.
       **Pass** = all five are *listed in the launcher* and each opens a window.
       Also: a `.html` file opens in Chromium (the HTTPS mime default now names
-      a browser that is present — #94's exact bug). Firefox is deliberately
-      absent; its absence is correct, not a miss.
-      **Known-not-covered:** a `.txt` file has no handler on the live ISO —
-      `text/plain` still names vscode, which the live medium never ships
-      (BACKLOG #119). Text Editor opening from the launcher is the pass here;
-      double-clicking a text file is #119's problem, so don't file it twice.
+      a browser that is present — #94's exact bug), and a `.txt` file opens in
+      **Text Editor** (`text/plain` falls through to
+      `org.gnome.TextEditor.desktop` after #119; vscode remains preferred on
+      template installs). Firefox is deliberately absent; its absence is
+      correct, not a miss.
 
 ## AMD dev box only
 - [ ] **#118 smartd still runs where drives DO have SMART** (this commit) — the