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/BACKLOG.md

@@ -55,31 +55,6 @@ template source. Pass = Chromium launches after installation and HTTP/HTTPS
 mime defaults resolve to `chromium-browser.desktop` (and the intended live-ISO
 browser posture is explicit).
 
-### 119. `text/plain` names vscode, which the live ISO does not ship
-
-Found while shipping #103 (2026-07-14), deliberately not fixed there — same
-bug class as #94, one mime key over. `modules/home/mime.nix:48` assigns
-`text/plain = code.desktop`, and vscode ships only in the **template**, never
-on the live ISO. GIO silently skips an entry whose desktop file is absent, so
-opening a text file in the live session finds no handler at all — the exact
-shape of the no-default-browser bug, and now the only one left of its kind
-since #103 put chromium on the live medium.
-
-Two ways it bites, and a fix should settle both: (1) the live session, which
-now ships `gnome-text-editor` (`org.gnome.TextEditor.desktop`) — the obvious
-live handler; (2) any installed machine where the user takes the template at
-its word and deletes the `vscode` line (opt-out = delete the line, per the
-option-surface convention), which silently breaks `text/plain` with no warning.
-
-The general trap is worth fixing once rather than per-key: a mime default
-naming a package the profile does not carry is invisible until a user
-double-clicks a file. `checks.live-baseline-apps` now guards exactly this for
-HTTPS (asserts the named .desktop is present, not merely named) — the cheap
-version of this item is extending that assertion to every key in
-`defaultApplications`, which would have caught both #94 and this. Pass = no
-mime default names a desktop entry the profile lacks, on the live ISO and on
-a template install, and a guard fails on a regression.
-
 ### 95. Live ISO/install: Ghostty does not open
 
 Reproduce from the launcher and a terminal, capture its stderr/journal, and