Fix models checkpoint directory structure for workflow caching

The workflow expects checkpoints at build/{mode}/checkpoints/ but the
code was creating them at build/{mode}/checkpoints/models/ due to using
packageName='models'.

Changed PACKAGE_NAME from 'models' to '' (empty string) to create flat
checkpoint structure, matching the pattern used by other builders like
node-smol-builder.

This ensures the workflow validation step can find:
- build/dev/checkpoints/finalized.json
- build/dev/checkpoints/finalized.tar.gz

Instead of looking in a non-existent subdirectory.

Author: jdalton

packages/models/scripts/build.mjs

@@ -82,7 +82,9 @@ const {
   outputFinalDir,
 } = getBuildPaths(BUILD_MODE)
 const DIST_MODE = outputFinalDir
-const PACKAGE_NAME = 'models'
+// Use empty string for packageName to create flat checkpoint structure
+// (workflow expects checkpoints at build/{mode}/checkpoints/, not build/{mode}/checkpoints/models/)
+const PACKAGE_NAME = ''
 
 const logger = getDefaultLogger()