terra: package rishot, a quickshell screenshot/annotation overlay

Not in nixpkgs — upstream ships a shell launcher + QML tree with no build
step, driven entirely by `qs -p <dir>`. Packaged as a stdenvNoCC derivation
that wraps the launcher with RISHOT_CONFIG_DIR (sidesteps its argv0-relative
self-lookup, which wrapProgram breaks) and its runtime deps on PATH.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

services/desktop/desktop-apps.nix

@@ -1,5 +1,9 @@
 { pkgs, lib, ... }:
 
+let
+  rishot = pkgs.callPackage ../../pkgs/rishot.nix { };
+in
+
 # Desktop applications for a workstation host (currently: terra). Split from
 # desktop-hyprland.nix, which owns the *session* (compositor, greeter, audio,
 # portals); this owns the things the user actually launches inside it.
@@ -28,6 +32,7 @@
     gitkraken
     jetbrains-toolbox
     kdePackages.dolphin
+    rishot
   ];
 
   fonts.packages = [ pkgs.nerd-fonts.departure-mono ];