Add startup update notification

[Widthdom]

Summary

• Check trusted nuget.org registration metadata for a newer listed stable nildiff release on eligible console starts.
• Cache successful checks for 20 hours and persist a one-hour failure backoff so NuGet outages do not add a two-second delay to every launch.
• Show three choices: update now, skip this run, or skip until the next release.
• Run only the fixed dotnet tool update --global nildiff process when the user explicitly chooses update.
• Centralize user-facing behavior and Windows/macOS/Linux examples in USER_GUIDE.md, with concise references from the other documents.

Behavior and impact

The update phase runs only when stdin, stdout, and stderr are all unredirected. Redirected, piped, and automated runs remain unchanged. Network, JSON, cache, timeout, and updater failures are best effort and cannot fail the requested CLI operation.

3. Skip until next version stores dismissedVersion in the user-local update-check.json. The same release remains suppressed across cache refreshes, while a later release prompts again. Existing clients through 1.22.0 cannot receive this startup behavior; it takes effect from the release containing this change.

Validation

• dotnet format FolderDiffIL4DotNet.sln --no-restore --verify-no-changes
• dotnet test FolderDiffIL4DotNet.Tests/FolderDiffIL4DotNet.Tests.csproj --configuration Release --no-restore --nologo
  • 2060 passed
  • 0 failed
  • 1 skipped
• UpdateNotificationServiceTests: 16 focused tests covering caching, stale refresh, failure backoff, trusted registration pages, all prompt choices, updater outcomes, and ProgramRunner stdout/stderr behavior.

Manual prompt verification

Create the user-local update-check.json with a latestVersion greater than the built application version, dismissedVersion set to null, and nextCheckAtUtc set to a future timestamp no more than 20 hours away. Then launch nildiff directly from an interactive console with all three standard streams unredirected. This exercises the cached path without contacting NuGet and displays choices 1, 2, and 3.