Expose SecretRef on the author surface for custom sandbox secrets

[czpython]

Problem

An app workspace can give its sandbox a custom secret by overriding Workspace.get_secret_refs(). For example, a per-rep OAuth service connection that the secrets proxy swaps in on one host:

from druks.sandbox.models import SecretRef  # internal module

class SalesforceWorkspace(Workspace):
    @classmethod
    async def get_secret_refs(cls, subject):
        return [SecretRef(name="sf_access_token", secret_id=subject.connection_id, host="org.my.salesforce.com")]

The hook works, but its return type is not public. druks.sandbox exports only Sandbox, so apps must import druks.sandbox.models. software_factory does the same. The author guide does not document get_secret_refs(), custom host entries, or the variable name, which is name.upper().

Tried

Adding SecretRef to druks/sandbox/__init__.py causes a circular import: druks.sandbox.client → druks.sandbox.host.

Proposal

• Make the type importable from a public namespace without the import cycle. This could be a lazy export, or a small datastructure like RequiredMcpServer that the workspace maps to the ORM row.
• Document get_secret_refs() in "Customize the workspace": the vault row, host, the derived variable, and the bearer header.
• Add it to the "Stable author imports" table.