feat: add app auto-update service and appcast tooling

[galz10]

Summary

Adds an app-managed auto-update service with deterministic state handling, surfaces update actions in the app UI, and introduces appcast generation/publishing tooling for release artifacts.

Details

• Added a new updater domain under idx0/Services/Updates:
  • state model + reducer (AppUpdateState, AppUpdateStatus, events/actions)
  • orchestration service (AppUpdateService) with startup delay + periodic polling
  • protocol seams for driver/scheduler/version/environment injection
  • Sparkle-backed driver boundary (SparkleUpdateDriver)
• Wired updater into app surfaces:
  • checkForUpdates command/action in shortcut + command registries
  • app menu “Check for Updates…” and context-aware update action
  • command palette update action
  • top bar status/action control
  • settings toggles/actions in advanced tab + inline advanced settings
• Extended settings and config:
  • AppSettings.autoCheckForUpdates with backward-compatible decode and schema bump
  • env support via IDX0_DISABLE_AUTO_UPDATE and IDX0_UPDATE_FEED_URL
• Added release/appcast tooling:
  • Sparkle package dependency and feed metadata in project config
  • scripts/generate-appcast.sh
  • scripts/publish-appcast.sh
  • scripts/manual-release.sh now emits appcast entry metadata JSON
• Added tests for updater reducer, action mapping, service orchestration, settings migration, command registration parity, and appcast stable/prerelease behavior.

Related Issues

N/A

How to Validate

1. Generate project files:
   • xcodegen generate
2. Run targeted updater/appcast tests:
   • xcodebuild -project idx0.xcodeproj -scheme idx0 -destination 'platform=macOS' -only-testing:idx0Tests/AppUpdateReducerTests -only-testing:idx0Tests/AppUpdateActionMapperTests -only-testing:idx0Tests/AppUpdateServiceTests -only-testing:idx0Tests/AppcastScriptTests test
3. Run full test suite:
   • xcodebuild -project idx0.xcodeproj -scheme idx0 -destination 'platform=macOS' test
4. Note on current baseline:
   • full suite still reports existing SessionServiceTests+Niri sizing failures (testNiriColumnResizePersistsPreferredWidths, testNiriDefaultTileSizesApplyOnlyToNewTiles), and these reproduce on main as well.

Pre-Merge Checklist

• Builds successfully (xcodebuild / Xcode)
• Added/updated tests (if needed)
• Updated relevant documentation and README (if needed)
• Noted breaking changes (if any)
• Tested on macOS