feat: add native platform installers for Windows, macOS, and Linux

Replace zip/tar archive distribution with platform-native installers:

- Windows: NSIS installer (.exe) with per-user install to
  %LOCALAPPDATA%\Programs\Aryx, Start Menu and Desktop shortcuts,
  Add/Remove Programs registration, and silent install support
- macOS: DMG disk image (.dmg) with drag-to-Applications experience
  via create-dmg
- Linux: Debian package (.deb) with /opt/aryx installation,
  /usr/bin/aryx symlink, desktop entry, hicolor icons, and
  libsecret-1-0 dependency declaration

Add scripts/create-installer.ts as the platform-dispatching entry
point. Update CI workflow to produce native installers instead of
archives. Add installerAssetName to ReleaseTarget interface.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

assets/installer/linux/aryx.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Name=Aryx
+Comment=Copilot-powered agent workflow orchestrator
+Exec=/opt/aryx/Aryx %U
+Icon=aryx
+Terminal=false
+Type=Application
+Categories=Development;
+StartupWMClass=Aryx