feat: add automatic updates for vibetyper, msty-studio, zellij-ps

- vibetyper/msty-studio: unversioned AppImage URLs → HEAD/Last-Modified
  check via custom update.sh, refreshes sha256 when upstream binary changes
- zellij-ps: switch to branch-tracking (master tip via Gitea API) since
  helper-scripts repo has no tags
- all three expose passthru.updateScript so the existing CI workflow
  discovers them via is_custom_update_script

pkgs/msty-studio/default.nix

@@ -1,3 +1,4 @@
+# last-modified: Wed, 24 Sep 2025 00:17:41 GMT
 {
   appimageTools,
   fetchurl,
@@ -9,6 +10,8 @@
 }: let
   pname = "msty-studio";
   version = "2.0.0-beta.4";
+  # Upstream publishes to an unversioned URL — ./update.sh watches Last-Modified
+  # and refreshes the sha256 when the binary changes. Bump `version` manually.
   src = fetchurl {
     url = "https://next-assets.msty.studio/app/alpha/linux/MstyStudio_x86_64.AppImage";
     sha256 = "sha256-zJcGK7QEL3ROgVJy13mMdY/437H3Zx8EwSXy7rEhV9w=";
@@ -34,6 +37,8 @@ in
               wrapProgram $out/bin/${pname} \
                       --prefix PATH : ${nodejs}/bin:${uv}/bin:${python3}/bin
     '';
+    passthru.updateScript = ./update.sh;
+
     meta = {
       description = "Msty Studio enables advanced, privacy‑preserving AI workflows entirely on your local machine.";
       license = lib.licenses.unfree;