CI: ship the tray GUI — in the archives and as tsunagi-gui deb/zst

Adds the tray application to the release:

- Every target's archive now also contains the tsunagi-tray binary. On Linux it
  is built against an old glibc baseline (via cargo-zigbuild) rather than static
  musl, because a GUI must dlopen libGL/X11/Wayland at runtime and a static musl
  binary cannot; the agent stays static musl. macOS and Windows build the tray
  natively.
- New GUI package variants for Linux: `tsunagi-gui` .deb and .pkg.tar.zst, built
  by the existing scripts with an extra tray argument. They install the agent
  (service, polkit, user — same as the headless packages) plus the tray binary,
  a desktop launcher (dist/linux/tsunagi-tray.desktop) and an icon
  (dist/linux/tsunagi.svg), so the GUI appears in the applications menu. The GUI
  package conflicts/replaces/provides the headless tsunagi so the two do not
  coexist.
- The tray finds the system service with no configuration: the library's default
  control-socket path now prefers /run/tsunagi/agent.sock when present (both the
  tray and CLI use this), falling back to the per-user path.

No control-protocol change. Builds, fmt and clippy clean on macOS; the Linux GUI
cross-build and the packages are exercised only by a real CI run.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/tsunagi/src/ipc/mod.rs

@@ -148,10 +148,17 @@ pub fn default_control_socket_path() -> PathBuf {
     }
     #[cfg(all(unix, not(target_os = "macos")))]
     {
+        // Prefer a running system service's socket when it is present, so a
+        // GUI or CLI reaches the packaged daemon with no configuration; fall
+        // back to the per-user location otherwise.
+        let system = PathBuf::from("/run/tsunagi/agent.sock");
+        if system.exists() {
+            return system;
+        }
         if let Ok(paths) = crate::config::StoragePaths::user_default() {
             control_socket_path(&paths.state_dir)
         } else {
-            PathBuf::from("/run/tsunagi/agent.sock")
+            system
         }
     }
 }