[mergify]

The asset cache used to store only apps///public, so any build output written elsewhere in the app was lost when a build was restored from the cache. It also skipped the steps bench build runs after bundling.

Build side (get_cached_app.py):

• Touch a per-app time marker before build_assets, then list every path under apps/ changed since the marker with find -cnewer.
• Skip node_modules (directories and the public/node_modules symlink) at any depth. Dependencies already come from the earlier yarn install, and the link is recreated on restore.
• Pack the list into added_files_.tar.gz inside the image, using --no-recursion so listed directories don't pull in unchanged contents.

Restore side (get_cached_app.py):

• extract_assets now overlays the changed files onto the app directory entry by entry instead of replacing public/ wholesale. Unchanged files are left alone, and existing symlinks are replaced, never written through.
• Restore the sites/assets/ symlink and node_modules link as before.
• Run bench compile-po-to-mo and the app's after_build hooks (new run_hooks.py), which bench build would otherwise have run.

Agent side (builder.py):

• upload_app_assets fetches the prebuilt tarball in a single get_archive call, unwraps it and uploads it as is, skipping the upload when nothing changed.
• Remove repack_public_folder, which is no longer needed.
• Copy run_hooks.py into the build context, with a test covering it.

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  This is an automatic backport of pull request Cache the changed build files instead of the public folder #664 done by Mergify.