Mark SemVer prereleases as GitHub prereleases

[levifig]

Problem

loaf release --post-merge creates a GitHub Release with gh release create but does not pass --prerelease when the released SemVer contains a prerelease component. The tag-triggered workflow has the same omission when it creates a missing release.

During v2.0.0-alpha.8 finalization, GitHub therefore classified the alpha as Latest instead of Pre-release. Publication was otherwise successful, and the release was corrected manually with:

gh release edit v2.0.0-alpha.8 --prerelease --latest=false

Evidence: release PR #118 and v2.0.0-alpha.8.

Expected behavior

• Versions with a SemVer prerelease component create or update a GitHub Release with isPrerelease: true and do not become Latest.
• Stable versions remain normal releases and may become Latest.
• Post-merge finalization and the tag workflow apply the same classification.
• Tests cover prerelease and stable publication command construction.

Boundary

This issue tracks the release-classification defect only. It does not change version-selection policy or the existing hard-cut lineage prerelease gate.