Create the service bus used by plugins to expose host-callable capabilities such as tools, model providers, permissions, commands, UI contributions, and integrations.

Acceptance criteria:

• Plugins can register named service interfaces.
• Service interfaces are versioned.
• Host can invoke plugin services by provider/interface/version/operation.
• Requests and responses use typed, serializable payloads.
• Service errors are structured and surfaced clearly.
• Multiple plugins can provide different services without hardcoded host behavior.