bakery: add license_file/desktop_file/data_archive manifest fields

Closes the packaging gap found while moving bread-ecosystem apps off
pacman onto bakery-only distribution: pacman's package() typically installs
a LICENSE file and, for GUI/onboarding apps, a .desktop entry and sometimes
a data directory (e.g. breadhelp's guide content). All three follow the
same download-verify-place pattern ConfigScaffold.example already
established:

- license_file -> ~/.local/share/licenses/<name>/LICENSE
- desktop_file -> ~/.local/share/applications/<name>.desktop
- data_archive -> a .tar.gz extracted to ~/.local/share/<name>/ (for
  arbitrary data too big/structured for a single file, via `tar`)

gen-index.sh parses all three from bakery.toml, hashes the artifact, and
now excludes them from the binaries-collection loop (previously undetected
gap: they'd have been swept in as fake "binaries" with no checksum, same
class of bug the existing .toml/.service/etc exclusions guard against).

Also registers breadhelp as a bakery-channel product.

registry/bread-ecosystem.toml

@@ -71,3 +71,8 @@ description = "Screenshot utility for the bread ecosystem"
 name = "bos-settings"
 repo = "Breadway/bos-settings"
 description = "System settings app for Bread OS"
+
+[[products]]
+name = "breadhelp"
+repo = "Breadway/breadhelp"
+description = "Onboarding and help center for Bread OS"