Add capability restrictions to plugin registry

[sebastientaggart]

Summary

Plugins currently receive a PluginRegistry with full access to actions, signals, state, events, and the orchestrator. A malicious or buggy plugin can crash the service or manipulate any state.

Proposal

• Introduce capability levels for plugins (e.g., read-only, state-only, full)
• Plugin config should declare required capabilities
• The registry passed to register() should be scoped to the declared capabilities
• A read-only plugin would receive a registry that omits write methods or raises on write attempts

Why

Defense in depth — even on a local service, a third-party plugin shouldn't need orchestrator access just to display a status widget. This also makes plugin intent explicit in configuration.

[sebastientaggart]

Resuming work on capability restrictions for the plugin registry.