desktop: the window installs itself (launcher entry, icon, default.nix)

app/linux/cst-agent-app.{desktop,svg} plus one CMake component ("app") in
app/CMakeLists.txt: `cmake --install --component app` puts the binary in bin/,
the entry in share/applications/ and the icon in hicolor/scalable/apps/. The
entry's file name stays the app_id Qt sets (QGuiApplication::applicationName),
or the launcher draws a generic icon; only the front end installs, the CLI
keeps coming from scripts/bootstrap_linux.sh.

default.nix is that install for nix users: nixpkgs' Qt 6 (qtwayland for a
Wayland session, qtsvg for Breeze's SVG icons), wrapQtAppsHook so the plugins
and QML modules resolve outside a nix-shell and survive nix-collect-garbage,
a filtered src (third_party/ and the build directories are gigabytes this
build never reads), and the version read from include/cst_version.h. doCheck
runs ctest minus test_sandbox, whose Landlock assertion cannot hold inside the
nix build sandbox (the other 40 pass there; make check runs all 41).

Verified: nix-build, nix profile add --file ., then --selftest and --uitest in
a clean environment (no nix-shell, no XDG_* from the session).

app/linux/cst-agent-app.desktop

@@ -0,0 +1,15 @@
+[Desktop Entry]
+Type=Application
+Name=CST-agent
+GenericName=Local coding agent
+Comment=The engine in a window: workspace is the directory it is started in
+Exec=cst-agent-app
+TryExec=cst-agent-app
+Icon=cst-agent-app
+Terminal=false
+StartupNotify=true
+# Qt puts the application name in the Wayland app_id / WM_CLASS, so this has to
+# stay the file's own name or the launcher draws a generic icon.
+StartupWMClass=cst-agent-app
+Categories=Development;Utility;
+Keywords=agent;ai;assistant;code;