docs: explain kit sets and environment file composition (#26221)

## Summary

The kit docs explain published sets and direct mixin composition but
don't help readers choose between a set and an environment file. Clarify
when to use each, link to environment files from the mixin composition
instructions, and note that separately composed OCI kits need distinct
image repository names.

@netlify /ai/sandboxes/customize/

Generated by Codex

content/manuals/ai/sandboxes/customize/_index.md

@@ -82,6 +82,14 @@ combined environment.
 If the set includes a workload, you run the published kit with `sbx run`.
 If it contains only mixins, you add it to a workload with `--kit`.
 
+To share a combination without publishing another kit, list the workload and
+mixins in a [sandbox environment file](/manuals/ai/sandboxes/configuration/environment-files.md).
+Docker Sandboxes composes those kits when it creates the sandbox. An environment
+file can also configure the workspace and host resources. Choose a set when you
+want one published reference with a fixed list of components; the set records
+their image digests when you build it. You can also use a set in an environment
+file.
+
 See [Use kits](/manuals/ai/sandboxes/customize/use-kits.md) for how to run kits
 and add mixins. To customize and publish your own combination, see
 [Compose a kit set](/manuals/ai/sandboxes/customize/author/kit-sets.md).

content/manuals/ai/sandboxes/customize/use-kits.md

@@ -135,6 +135,11 @@ feature, Docker Sandboxes can reject the combination.
 
 To publish your combination as one reference, see
 [Compose a kit set](/manuals/ai/sandboxes/customize/author/kit-sets.md).
+To keep the kits separate in a shared sandbox configuration, list them in a
+[sandbox environment file](/manuals/ai/sandboxes/configuration/environment-files.md).
+Give each published kit its own image repository name. Different tags of the
+same repository have the same kit name, and Docker Sandboxes rejects duplicate
+names in a composition.
 
 ### Change a sandbox's mixins