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/CMakeLists.txt

@@ -14,6 +14,10 @@ project(cst-agent-app LANGUAGES C CXX VERSION 0.1.0)
 # Inside the repo build (the parent defines the cstcore target, shared with the CLI):
 #   cmake -B build -DCST_BUILD_APP=ON && cmake --build build --target cst-agent-app
 #
+# Desktop install (the window, its launcher entry and the icon; the CLI has
+# its own installer) -- default.nix installs exactly this component:
+#   cmake --install build --component app --prefix ~/.local
+#
 # Extra QML import paths, e.g. a Kirigami installed away from the Qt prefix:
 #   -DCST_APP_QML_IMPORT=...   (added to QML2_IMPORT_PATH / the app's runtime)
 
@@ -119,6 +123,23 @@ if(CST_APP_GUI)
       set_target_properties(cst-agent-app PROPERTIES WIN32_EXECUTABLE TRUE)
     endif()
 
+    # A desktop install. The icon goes where the theme layout says, so the
+    # entry's Icon= name finds it, and the entry's file name is the app_id Qt
+    # sets (QGuiApplication::applicationName) -- rename one and the launcher
+    # loses the icon. All of it in the "app" component: the CLI keeps coming
+    # from scripts/bootstrap_linux.sh, and two `cst-agent` in PATH is how a
+    # version nobody meant to run gets run.
+    if(NOT ANDROID)
+      include(GNUInstallDirs)
+      install(TARGETS cst-agent-app
+              RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT app)
+      install(FILES linux/cst-agent-app.desktop
+              DESTINATION ${CMAKE_INSTALL_DATADIR}/applications COMPONENT app)
+      install(FILES linux/cst-agent-app.svg
+              DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/scalable/apps
+              COMPONENT app)
+    endif()
+
     # --- Android (APK) -----------------------------------------------------
     # The engine is compiled in (top-level CMakeLists: -DCST_BUILD_TUI=OFF,
     # -DCST_BUILD_WEBUI=OFF, so src/webui_stub.c stands in for the server and

app/README.md

@@ -45,6 +45,31 @@ navigation drawer) so swapping in `org.kde.kirigami` later is a QML-only
 change. Point the app at a Kirigami outside the Qt prefix with
 `-DCST_APP_QML_IMPORT=/path/to/qml`.
 
+## Install (Linux desktop)
+
+The window installs itself: the binary, a launcher entry and the icon the
+launcher draws for it. All three are one CMake component (`app`), so the CLI
+keeps coming from `scripts/bootstrap_linux.sh` and the profile never holds two
+`cst-agent` of different vintages; the files live in `app/linux/`.
+
+```sh
+# NixOS / nix users: PATH + menu entry + icon. The Qt apps hook bakes the
+# plugin and QML module paths into the wrapper, so the app runs outside a
+# nix-shell and nix-collect-garbage cannot pull Qt out from under it.
+nix-build                       # ./result/bin/cst-agent-app, run it in place
+nix profile add --file .        # or: ./result/bin/cst-agent-app
+nix profile remove cst-agent    # uninstall
+
+# anywhere with Qt 6 (Quick/Qml/QuickControls2) and libcurl:
+cmake -B build-app -DCST_BUILD_APP=ON -DCMAKE_BUILD_TYPE=Release
+cmake --build build-app -j
+cmake --install build-app --component app --prefix ~/.local
+```
+
+Launched from a menu, the workspace is the directory the app starts in
+(`$HOME` there), so sessions land in `~/.agent/sessions`; pass
+`--workspace <dir>` to pin another one.
+
 ## Why in one process
 
 The engine is a library, not a daemon: `libcstcore.a` (Makefile) / the
@@ -408,6 +433,7 @@ so nothing about it is visible when the app is idle.
       file card's open/share through the app's FileProvider
 - [x] a turn keeps running with the app off screen (Android foreground service
       + wake lock; nothing to do on a desktop)
+- [x] desktop install: launcher entry + icon, `nix profile add --file .`
 - [ ] a real turn driven through the window (needs a provider key)
 - [ ] SAF file picker, the back gesture, the icon glyphs on the phone
 - [ ] Kirigami (the rest of the Breeze look: colors, controls), tray,

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;

app/linux/cst-agent-app.svg

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- The launcher icon: the prompt the CLI shows, on a tile in the window's own
+     palette (app/qml/Theme.qml). Installed under hicolor/scalable, so the
+     desktops that want raster sizes render it themselves. -->
+<svg xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
+  <rect x="6" y="6" width="116" height="116" rx="26" ry="26"
+        fill="#15181a" stroke="#343b41" stroke-width="5"/>
+  <path d="M36 42 L56 64 L36 86" fill="none" stroke="#3daee9" stroke-width="12"
+        stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M69 88 H94" fill="none" stroke="#eff0f1" stroke-width="12"
+        stroke-linecap="round"/>
+</svg>