fix(release): generate Velopack delta packages

[mchave3]

Summary

• Download the latest matching Velopack release before packaging so vpk pack can generate delta packages.
• Pass GITHUB_TOKEN to the release packaging step for authenticated GitHub release downloads.
• Keep copied release assets scoped to the current package version while preserving Velopack output metadata for upload.

Reason

Velopack only creates delta packages when the previous full package is available in the packaging output directory. The release script cleared that directory before packing, so releases only produced full packages.

Main changes

• Added optional previous-release download support to Publish-FoundryVelopack.ps1.
• Added -SkipPreviousReleaseDownload for local packaging scenarios that should not prefetch the latest release.
• Updated release asset cleanup/copy filters for both full and delta packages.
• Exposed GITHUB_TOKEN to the release asset build step.

Testing notes

• Parsed scripts/Publish-FoundryVelopack.ps1 with the PowerShell parser.
• Ran git diff --check.
• Generated local x64 Velopack assets for 26.5.16-build.2; verified full, delta, and MSI outputs.
• Generated local arm64 Velopack assets for 26.5.16-build.2; verified full, delta, and MSI outputs.
• Generated a local J-1 x64 MSI for 26.5.15-build.2 with -SkipPreviousReleaseDownload.

[Copilot]

Pull request overview

Enables Velopack delta package generation in release builds by downloading the previous release into the packaging output directory before vpk pack. Also adjusts asset cleanup/copy filters to include delta packages and threads GITHUB_TOKEN into the release workflow step for authenticated downloads.

Changes:

• Add VelopackFeedUrl, GitHubToken, and SkipPreviousReleaseDownload parameters to Publish-FoundryVelopack.ps1 and invoke vpk download github before packing.
• Extend release-asset cleanup to include *-delta.nupkg files and scope the post-pack copy filter to the current PackVersion.
• Expose GITHUB_TOKEN to the Build release assets step in release.yml.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
scripts/Publish-FoundryVelopack.ps1	Download previous release into $releaseDir so vpk can produce delta packages; update asset filter patterns for delta nupkgs and version-scoped copy.
.github/workflows/release.yml	Pass github.token to the release packaging step so the script can authenticate GitHub downloads.

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