Document models checkpoint structure and fix packageName usage

Clarifies that packageName should be '' (empty string) for flat checkpoint
structure that matches workflow caching expectations. Adds inline comments
explaining why each call uses packageName=''.

Changes:
- Remove PACKAGE_NAME constant (was confusing)
- Add comments above each packageName='' explaining workflow requirement
- Makes it clear this is intentional for workflow caching compatibility

No functional changes - just better documentation to prevent future mistakes.

Author: jdalton

packages/models/scripts/build.mjs

@@ -82,9 +82,6 @@ const {
   outputFinalDir,
 } = getBuildPaths(BUILD_MODE)
 const DIST_MODE = outputFinalDir
-// 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()
 
@@ -113,11 +110,12 @@ const MODEL_SOURCES = {
  * Download model from Hugging Face.
  */
 async function downloadModel(modelKey) {
+  // Use empty packageName for flat checkpoint structure (workflow caching requirement)
   return downloadModelImpl({
     modelKey,
     modelSources: MODEL_SOURCES,
     buildDir: BUILD,
-    packageName: PACKAGE_NAME,
+    packageName: '',
     modelsDir: MODELS,
     forceRebuild: FORCE_BUILD,
   })
@@ -127,11 +125,12 @@ async function downloadModel(modelKey) {
  * Convert model to ONNX if needed.
  */
 async function convertToOnnx(modelKey) {
+  // Use empty packageName for flat checkpoint structure (workflow caching requirement)
   return convertToOnnxImpl({
     modelKey,
     modelSources: MODEL_SOURCES,
     buildDir: BUILD,
-    packageName: PACKAGE_NAME,
+    packageName: '',
     modelsDir: MODELS,
     forceRebuild: FORCE_BUILD,
   })
@@ -147,11 +146,12 @@ async function convertToOnnx(modelKey) {
  * Results in significant size reduction with minimal accuracy loss.
  */
 async function quantizeModel(modelKey, quantLevel) {
+  // Use empty packageName for flat checkpoint structure (workflow caching requirement)
   return quantizeModelImpl({
     modelKey,
     quantLevel,
     buildDir: BUILD,
-    packageName: PACKAGE_NAME,
+    packageName: '',
     modelsDir: MODELS,
     forceRebuild: FORCE_BUILD,
   })
@@ -294,7 +294,7 @@ async function main() {
     if (outputMissing) {
       logger.step('Output artifacts missing - cleaning stale checkpoints')
     }
-    await cleanCheckpoint(BUILD, PACKAGE_NAME)
+    await cleanCheckpoint(BUILD)
   }
 
   // Create directories.
@@ -331,7 +331,6 @@ async function main() {
     // Downloaded checkpoint: Archive all downloaded models
     await createCheckpoint(
       BUILD,
-      PACKAGE_NAME,
       'downloaded',
       async () => {
         // Smoke test: Verify models directory exists and has models
@@ -353,7 +352,6 @@ async function main() {
     // Converted checkpoint: Archive all converted models (same directory)
     await createCheckpoint(
       BUILD,
-      PACKAGE_NAME,
       'converted',
       async () => {
         // Smoke test: Verify at least one ONNX file exists
@@ -379,7 +377,6 @@ async function main() {
     // Quantized checkpoint: Archive all quantized models (same directory)
     await createCheckpoint(
       BUILD,
-      PACKAGE_NAME,
       'quantized',
       async () => {
         // Smoke test: Verify at least one quantized model exists
@@ -426,7 +423,6 @@ async function main() {
     // Create finalized checkpoint for caching
     await createCheckpoint(
       BUILD,
-      PACKAGE_NAME,
       'finalized',
       async () => {
         // Smoke test: Verify final output files exist