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>

tests/scripts/releaseTarget.test.ts

@@ -15,6 +15,7 @@ describe('resolveReleaseTarget', () => {
       dotnetRuntime: 'win-x64',
       outputDirectoryName: 'Aryx-windows-x64',
       archiveBaseName: 'Aryx-windows-x64',
+      installerAssetName: 'Aryx-windows-x64-setup.exe',
       sidecarExecutableName: 'Aryx.AgentHost.exe',
       packagedExecutableName: 'Aryx.exe',
     });
@@ -28,6 +29,7 @@ describe('resolveReleaseTarget', () => {
       dotnetRuntime: 'osx-arm64',
       outputDirectoryName: 'Aryx-macos-arm64',
       archiveBaseName: 'Aryx-macos-arm64',
+      installerAssetName: 'Aryx-macos-arm64.dmg',
       sidecarExecutableName: 'Aryx.AgentHost',
       appBundleName: 'Aryx.app',
     });
@@ -41,6 +43,7 @@ describe('resolveReleaseTarget', () => {
       dotnetRuntime: 'linux-x64',
       outputDirectoryName: 'Aryx-linux-x64',
       archiveBaseName: 'Aryx-linux-x64',
+      installerAssetName: 'aryx-linux-x64.deb',
       sidecarExecutableName: 'Aryx.AgentHost',
       packagedExecutableName: 'Aryx',
     });