Converge Provider

This crate defines the provider-facing capability contract for Converge. It is deliberately separate from pack authoring:

• chat traits and request/response types live here
• [Backend] and [BackendKind] describe provider identity
• [Capability] describes what a provider can do
• [BackendRequirements] and [BackendSelector] drive routing
• [BackendError] is the generic provider error surface

Suggestor and invariant authoring do not live here. Those contracts belong to converge-pack.