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>

scripts/releaseTarget.ts

@@ -11,6 +11,7 @@ export interface ReleaseTarget {
   readonly dotnetRuntime: `${string}-${SupportedArch}`;
   readonly outputDirectoryName: string;
   readonly archiveBaseName: string;
+  readonly installerAssetName: string;
   readonly sidecarExecutableName: string;
   readonly packagedExecutableName?: string;
   readonly appBundleName?: string;
@@ -43,6 +44,7 @@ export function resolveReleaseTarget(
         dotnetRuntime: `win-${supportedArch}`,
         outputDirectoryName: archiveBaseName,
         archiveBaseName,
+        installerAssetName: `${archiveBaseName}-setup.exe`,
         sidecarExecutableName: 'Aryx.AgentHost.exe',
         packagedExecutableName: `${productName}.exe`,
       };
@@ -58,6 +60,7 @@ export function resolveReleaseTarget(
         dotnetRuntime: `osx-${supportedArch}`,
         outputDirectoryName: archiveBaseName,
         archiveBaseName,
+        installerAssetName: `${archiveBaseName}.dmg`,
         sidecarExecutableName: 'Aryx.AgentHost',
         appBundleName: `${productName}.app`,
       };
@@ -73,6 +76,7 @@ export function resolveReleaseTarget(
         dotnetRuntime: `linux-${supportedArch}`,
         outputDirectoryName: archiveBaseName,
         archiveBaseName,
+        installerAssetName: `aryx-linux-${supportedArch}.deb`,
         sidecarExecutableName: 'Aryx.AgentHost',
         packagedExecutableName: productName,
       };