Update error message in packages.ts to remove 'lowercase' requirement

[mgoldsborough]

Context

After merging #41 (fix-announce-bug-2), the packages.ts publish route normalizes package names to lowercase before validation. However, the error message at apps/registry/src/routes/packages.ts:120 still says "lowercase alphanumeric characters and hyphens" — implying the user must provide lowercase input, which is no longer true.

The other two routes (bundles.ts, skills.ts) were updated in #41 to say "alphanumeric characters and hyphens" (without "lowercase"). This route should match.

Fix

In apps/registry/src/routes/packages.ts, update the validation error string to remove "lowercase" — aligning with the wording in bundles.ts and skills.ts.

One-line change.