Add Sparkle auto-update support

[nniel-ape]

Closes #78

Summary

• Integrate Sparkle 2.x via Swift Package Manager for automatic in-app updates
• Add "Check for Updates…" menu item under the app menu
• Add Settings window with update preferences (auto-check, auto-download, version info)
• Extend release workflow to EdDSA-sign builds, generate appcast.xml, and deploy it to GitHub Pages

New Files

File	Purpose
GitClient/Info.plist	SUFeedURL (appcast) and SUPublicEDKey (placeholder)
GitClient/Models/Observables/CheckForUpdatesViewModel.swift	Observable wrapper around SPUUpdater state
GitClient/Views/SettingsView.swift	Settings window with update toggles and "Check for Updates" button

Modified Files

File	Changes
GitClientApp.swift	Initialize SPUStandardUpdaterController, add menu command & Settings scene
project.pbxproj / Package.resolved	Add Sparkle SPM dependency
.github/workflows/release.yml	Sign with Sparkle, generate appcast, deploy to gh-pages

Maintainer Setup

Before the first release with auto-updates, complete these one-time steps:

1. Generate an EdDSA keypair

Clone Sparkle and run:

swift package --package-path /path/to/Sparkle resolve
/path/to/Sparkle/.build/artifacts/Sparkle/bin/generate_keys

This prints a public key and stores the private key in your Keychain. Export the private key with generate_keys -x.

2. Replace the public key placeholder

In GitClient/Info.plist, replace PLACEHOLDER_EDDSA_PUBLIC_KEY with the public key from step 1.

3. Add the repository secret

Go to Settings → Secrets and variables → Actions and add:

• Name: SPARKLE_EDDSA_PRIVATE_KEY
• Value: the private key from step 1

4. Enable GitHub Pages

Go to Settings → Pages and set:

• Source: Deploy from a branch
• Branch: gh-pages / / (root)

The appcast will be served at https://maoyama.github.io/Changes/appcast.xml.

Test Plan

• Project builds successfully with Sparkle dependency
• All 34 existing tests pass
• Verify "Check for Updates…" menu item appears under app menu
• Verify Settings window opens with update preferences
• After maintainer setup: verify release workflow signs and publishes appcast