fix: stackbilt scaffold reads from run output, not only architect cache

[stackbilt-admin]

Bug

stackbilt scaffold reads .charter/last-build.json written by stackbilt architect. stackbilt run writes files directly to disk but does NOT write this cache file. So stackbilt run && stackbilt scaffold doesn't compose — scaffold has nothing to read.

Fix

Standardise on a single cache contract written by both architect and run:

.charter/last-build.json  {
  intention: string
  pattern: string
  classification: ClassifyResult
  governance: GovernanceDocs
  files?: ScaffoldFile[]   // present after run, absent after architect
  createdAt: string
}

Both commands write this shape. scaffold reads it and materialises files from files[] if present, or calls generateFiles() from the cached classification if not.

Related

#4 (inference-free scaffold wiring) depends on this cache contract being stable.

[stackbilt-admin]

Fixed in PR #8: unified cache contract implemented. architect writes { intention, pattern, classification, governance, createdAt }. run writes the same shape plus files[]. scaffold.ts reads all three shapes (new, architect-no-files, legacy BuildResult).