Support stable-only latest package policy

[stiwicourage]

Summary

Support policy-based handling of the floating latest package alias so preview releases do not replace the production-safe stable alias.

Problem

Some consumers always deploy latest in production. That alias should continue to point to the newest stable release.

Preview users should opt in explicitly by using a versioned preview package name instead of consuming latest.

Proposed configuration

Allow Package.Latest to accept these values:

• "never" — never create or update a floating alias
• "stable" — only stable releases update latest
• "always" — both stable and preview releases update latest

Recommended configuration:

"Package": {
  "Latest": "stable"
}

Backward compatibility

Temporarily support the existing boolean values:

• true => "always"
• false => "never"

The string values should be the preferred configuration going forward.

Documentation and deprecation approach

• Docs and examples should show only the string-based policy values.
• CHANGELOG.md should mark boolean Package.Latest values as deprecated.
• Add clear TODO: markers where legacy boolean handling exists so it is easy to remove in the next major version.

Acceptance criteria

• Package.Latest = "stable" publishes the versioned package for all releases, but only updates latest for stable releases.
• Package.Latest = "always" updates latest for both stable and preview releases.
• Package.Latest = "never" disables the floating alias entirely.
• Legacy booleans still work for now, with true mapped to "always" and false mapped to "never".
• Docs and examples present only the string-based values as the supported configuration style.
• CHANGELOG.md records boolean deprecation.
• Code contains TODO markers for removing legacy boolean support in the next major version.

Why this helps

• latest stays trustworthy for production.
• Preview consumers must opt in explicitly.
• Nova gets a clearer and more extensible configuration model without an immediate breaking change.