fix(publish): add --skip-duplicate so nuget push is idempotent (#108)

Without --skip-duplicate, a single failed push aborts the whole publish
target, and a rerun then fails with HTTP 409 ("already exists") on
every package that did succeed the first time. That makes release.yml
non-recoverable from any partial-failure state, which we hit twice
today (#107 → SolutionPersistence prefix-reservation, then again
mid-key-permission-debug).

With --skip-duplicate, the push step returns success when a version
is already on nuget.org. Reruns are now safe and only the genuinely
missing packages get uploaded.

Wired via IPublish.PushSettings override (Configure<DotNetNuGetPushSettings>),
which composes on top of the existing PushSettingsBase in
Fallout.Components.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ChrisonSimtian

build/Build.cs

@@ -142,6 +142,12 @@ IEnumerable<AbsolutePath> IPublish.PushPackageFiles
         => From<IPack>().PackagesDirectory.GlobFiles("*.nupkg")
             .Where(x => !x.NameWithoutExtension.StartsWith("Nuke.", StringComparison.OrdinalIgnoreCase));
 
+    // `--skip-duplicate` makes nuget push idempotent: if a version is already on the feed,
+    // skip it instead of erroring. Lets us rerun release.yml safely when a single package
+    // fails mid-batch without nuking the whole pipeline on retry.
+    Configure<DotNetNuGetPushSettings> IPublish.PushSettings => _ => _
+        .EnableSkipDuplicate();
+
     IEnumerable<AbsolutePath> NuGetPackageFiles
         => From<IPack>().PackagesDirectory.GlobFiles("*.nupkg");