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

assets/installer/windows.nsi

@@ -0,0 +1,112 @@
+!include "MUI2.nsh"
+
+;--- Product metadata (passed via /D defines from the build script) ---
+!ifndef PRODUCT_NAME
+  !define PRODUCT_NAME "Aryx"
+!endif
+!ifndef PRODUCT_VERSION
+  !define PRODUCT_VERSION "0.0.0"
+!endif
+!ifndef PRODUCT_PUBLISHER
+  !define PRODUCT_PUBLISHER "David Kaya"
+!endif
+!ifndef SOURCE_DIR
+  !error "SOURCE_DIR must be defined (path to the packaged app directory)."
+!endif
+!ifndef OUTPUT_PATH
+  !error "OUTPUT_PATH must be defined (path to the output installer .exe)."
+!endif
+
+;--- Installer attributes ---
+Name "${PRODUCT_NAME}"
+OutFile "${OUTPUT_PATH}"
+InstallDir "$LOCALAPPDATA\Programs\${PRODUCT_NAME}"
+InstallDirRegKey HKCU "Software\${PRODUCT_NAME}" "InstallDir"
+RequestExecutionLevel user
+SetCompressor /SOLID lzma
+
+;--- Version info embedded in the installer EXE ---
+VIProductVersion "${PRODUCT_VERSION}.0"
+VIAddVersionKey "ProductName" "${PRODUCT_NAME}"
+VIAddVersionKey "ProductVersion" "${PRODUCT_VERSION}"
+VIAddVersionKey "FileDescription" "${PRODUCT_NAME} Setup"
+VIAddVersionKey "FileVersion" "${PRODUCT_VERSION}"
+VIAddVersionKey "CompanyName" "${PRODUCT_PUBLISHER}"
+VIAddVersionKey "LegalCopyright" "Copyright ${PRODUCT_PUBLISHER}"
+
+;--- MUI configuration ---
+!define MUI_ABORTWARNING
+
+;--- Installer pages ---
+!insertmacro MUI_PAGE_WELCOME
+!insertmacro MUI_PAGE_DIRECTORY
+!insertmacro MUI_PAGE_INSTFILES
+!define MUI_FINISHPAGE_RUN "$INSTDIR\${PRODUCT_NAME}.exe"
+!insertmacro MUI_PAGE_FINISH
+
+;--- Uninstaller pages ---
+!insertmacro MUI_UNPAGE_CONFIRM
+!insertmacro MUI_UNPAGE_INSTFILES
+
+;--- Language ---
+!insertmacro MUI_LANGUAGE "English"
+
+;--- Installer section ---
+Section "Install"
+  ; Close any running instance
+  ExecWait 'taskkill /F /IM ${PRODUCT_NAME}.exe' $0
+
+  SetOutPath "$INSTDIR"
+  File /r "${SOURCE_DIR}\*.*"
+
+  ; Write uninstaller
+  WriteUninstaller "$INSTDIR\Uninstall.exe"
+
+  ; Start Menu shortcut
+  CreateDirectory "$SMPROGRAMS\${PRODUCT_NAME}"
+  CreateShortcut "$SMPROGRAMS\${PRODUCT_NAME}\${PRODUCT_NAME}.lnk" "$INSTDIR\${PRODUCT_NAME}.exe"
+  CreateShortcut "$SMPROGRAMS\${PRODUCT_NAME}\Uninstall ${PRODUCT_NAME}.lnk" "$INSTDIR\Uninstall.exe"
+
+  ; Desktop shortcut
+  CreateShortcut "$DESKTOP\${PRODUCT_NAME}.lnk" "$INSTDIR\${PRODUCT_NAME}.exe"
+
+  ; Add/Remove Programs registry entry
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "DisplayName" "${PRODUCT_NAME}"
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "UninstallString" '"$INSTDIR\Uninstall.exe"'
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "QuietUninstallString" '"$INSTDIR\Uninstall.exe" /S'
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "InstallLocation" "$INSTDIR"
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "DisplayIcon" "$INSTDIR\${PRODUCT_NAME}.exe"
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "Publisher" "${PRODUCT_PUBLISHER}"
+  WriteRegStr HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "DisplayVersion" "${PRODUCT_VERSION}"
+  WriteRegDWORD HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "NoModify" 1
+  WriteRegDWORD HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}" \
+    "NoRepair" 1
+
+  ; Store install directory for future upgrades
+  WriteRegStr HKCU "Software\${PRODUCT_NAME}" "InstallDir" "$INSTDIR"
+SectionEnd
+
+;--- Uninstaller section ---
+Section "Uninstall"
+  ; Close any running instance
+  ExecWait 'taskkill /F /IM ${PRODUCT_NAME}.exe' $0
+
+  ; Remove application files
+  RMDir /r "$INSTDIR"
+
+  ; Remove shortcuts
+  RMDir /r "$SMPROGRAMS\${PRODUCT_NAME}"
+  Delete "$DESKTOP\${PRODUCT_NAME}.lnk"
+
+  ; Remove registry entries
+  DeleteRegKey HKCU "Software\Microsoft\Windows\CurrentVersion\Uninstall\${PRODUCT_NAME}"
+  DeleteRegKey HKCU "Software\${PRODUCT_NAME}"
+SectionEnd