fix(updater): publish one merged S3 auto-update manifest per OS

[sproctor]

🚀 Description

Claude generated description of changes:

Nucleus runs a separate electron-builder invocation per TargetFormat, and each updatable run publishes its own single-artifact <channel><osSuffix>.yml. Because the manifest name is keyed only on the OS, every format of the same OS publishes to the same S3 key, so the last writer clobbers the others — e.g. the deb run overwrites the AppImage entry, and an AppImage client then hits NoMatchingFileException (on macOS a dmg-only latest-mac.yml breaks Squirrel.Mac updates entirely).

Fix: always set publishAutoUpdate: false for S3 so electron-builder uploads the artifacts but not the manifests, then a new merge task discovers the per-format manifests for the current OS, unions their files: entries (UpdateYmlMerger), and uploads a single manifest via aws s3 cp. A lone manifest passes through verbatim to preserve electron-builder's exact metadata.

• TargetFormat.producesUpdateManifest gates registering the merge task.
• Upload mirrors electron-builder's publish decision (publish mode + CI tag), fails loudly if a manifest is expected but missing or the aws CLI is absent.
• Tested: merge/discovery, publish-flag/tag logic, s3 key, and the publishAutoUpdate:false config emission.

Scope: S3 only. GitHub releases are already consolidated in CI (generate-update-yml).

📄 Motivation and Context

Fixes #253

🧪 How Has This Been Tested?

Added tests.

📦 Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.