Bulk set or delete workspace secrets

Sets or deletes multiple workspace secrets in one request, mirroring the upstream SecretUpsert contract. A null value deletes the key; a non-null value sets it. Values are never returned. Secrets are a single shared namespace per workspace. Any agent (triggered by any member) can use them, and any member whose role grants workspaces:manage-secrets (Workspace Admins by default) can overwrite or delete them. Setting an existing name replaces its value for everyone. Treat them as shared team credentials, not personal keys.