Clarify registry credential use in Sandboxes (#26450)

Update the host-only registry credential description to refer to `sbx`
as a whole, covering pulls authenticated by the daemon. The credential
storage commands and scope guidance are unchanged.

Validation: Markdown formatting and scoped rumdl/Vale checks passed.

content/manuals/ai/sandboxes/configuration/credentials.md

@@ -620,7 +620,7 @@ passing neither:
 sbx secret set [--all-sandboxes | --sandbox SANDBOX] --registry HOST
 ```
 
-- **Host-only** (no scope flag): the `sbx` CLI uses it to pull templates
+- **Host-only** (no scope flag): `sbx` uses it to pull templates
   and kits when creating a sandbox. The credential stays on the host and is
   never available inside the sandbox.
 - **All sandboxes** (`--all-sandboxes`): same as host-only, plus the proxy