Fetching Subscriptions (subscriptions/list)

[leogdion]

[leogdion]

Status Update: The listSubscriptions operation exists in Sources/MistKit/Generated/Client.swift (line 2006) but is not exposed in the public CloudKitService API.

What needs to be done:

1. Create a public wrapper method in CloudKitService+Operations.swift
2. Define a SubscriptionInfo type similar to RecordInfo/ZoneInfo
3. Add documentation explaining subscription types (query, zone, database)
4. Add tests for listing subscriptions

Generated operation signature:

internal func listSubscriptions(_ input: Operations.listSubscriptions.Input) 
  async throws -> Operations.listSubscriptions.Output

Part of the subscriptions API (#49, #50, #51) - enables push notification management.