standard: move chunkah to fedora-only manifest

The chunkah package is not yet available in CentOS Stream/RHEL,
so having it in the shared standard/manifest.yaml breaks backports
(see https://gitlab.com/redhat/centos-stream/containers/bootc/-/merge_requests/1034).

Move it to the top-level standard.yaml which is Fedora-specific and
not backported.

Assisted-by: OpenCode (Claude Opus 4.6)

standard.yaml

@@ -5,3 +5,7 @@ include:
 packages:
   # Make Ansible "package" builtin work by default
   - python3-libdnf5
+  # Content-based container layer splitting for rechunking
+  # Note we should be able to move this back to the base standard/manifest.yaml
+  # once chunkah is in CentOS Stream and RHEL.
+  - chunkah

standard/manifest.yaml

@@ -45,8 +45,6 @@ packages:
   - python3-rpm
   # Used by admins interactively
   - man-db
-  # Content-based container layer splitting for rechunking
-  - chunkah
 
 # These are random architecture-specific packages
 packages-x86_64: